NOTE: I'm not part of any development team, so don't take what is written below as if it were part of the game.
I have some ideas, but I have no clue if these are practicle or implementable for your game.
A number of stats would be used in determining the various "powers" of a creature:
Hitpoints (HP): How much damage a creature can take. It is possible to have high hitpoints and low armor, like a soft-skinned Sauropod. An issue with hitpoints is the issue of size; a creature the size of a raptor could have 100 hitpoint, but to justify that, a Sauropod would have to have literally thousands of hitpoints to be even remotely realistic. To make up for this, lower-HP creatures could attack in large numbers. An average human would have between 100 and 200 hitpoints, whereas the strongest human who ever lived would have 300. When hitpoints run out, you die.
Armor: How tough a creature is, this stat reduces damage taken, but if the attack stat is greater than the armor stat, the difference deals damage to the hitpoints. A smaller, but better protected dinosaur like an anklyosaurus could conceivably survive a bite which would kill a much larger dinosaur. Armor and hitpoints are two different things; large doesn't necessarily mean tough, small doesn't necessarily mean soft.
Attack(s): Pretty much self-explanatory, this is how much damage a creature can deal to another. Damage could be given a range (like a bite dealing 5-15 damage) and then would have to be subtracted from the armor rating. Some creatures could have multiple kinds of attacks; Redclaw, for instance, could have the ability to bite, kick, bash (with his head), and use a tail-sweep. Attacks cost stamina, as shown next to the damage caused by an attack in parentheses.
Agility: The ability to avoid attacks and to successfully hit with an attack. Whether or not this is to be distinct from speed is a serious question, but it probably ought to be; a Triceratops can run quickly in a sprint, but is by no means agile. "Defense" would be a function of agility and speed. A random number would have to be generated and added in order to make the system work when comparing agility between two creatures, otherwise an attack would always hit or always miss the other creature. Let's just say that this number is between 1-20.
Speed: How fast one runs/swims/flies. Should be important in determining the number of attacks one can make in a given amount of time, and how likely one can escape from a battle or how likely one could catch up with someone trying to flee. Creatures with higher speed attack first in a round. The chances of attacking more than once in a round are determined by one's speed, but I'll go into multiple attacks in a later suggestion.
Stamina: How long a character can exert him/herself. Measured in number form; note that size and stamina aren't the same thing. Attacking has a stamina cost, as do other actions not under this particular proposed set of rules. Resting restores stamina. When out of stamina, a character is exhausted and cannot move or attack.
Here are some very rough examples:
Redclaw
HP: 3500
Armor: 12 (above-average)
Agility: 8 (below average)
Speed: 10 (average)
Stamina: 1000 (average)
Attacks:
Bite: 100-250 dmg (20 stm)
Kick: 50-75 (10 stm)
Tail: 20-30 (30 stm)
Bash: 40-65 (5 stm)
These are the stats for a big, tough predator. Now let's see a prediction for a smaller creature, one that Redclaw could expect to kill with one bite.
Ruby's Mother
HP: 100
Armor: 8 (below-average)
Agility: 14 (well above average)
Speed: 16 (exceptional)
Stamina: 800 (below average)
Attack:
Kick: 5-15 (10 stm)
Ruby's mother is by no means an excellent combatant, but like her daughter, she is fast on her feet, and well-balanced. She has no teeth, but the same claws and muscles in her feet that enable her to run quickly give her a good kick attack for her size. Note that the kick assumes that oviraptors have sharp claws. These attacks were made so that a smaller creature like herself could not do much damage to much larger, tougher opponents. Let's look at her mate to see some slight differences.
Ruby's Father
HP: 135
Armor: 9 (below-average)
Agility: 12 (above-average)
Speed: 16 (exceptional)
Stamina: 1000 (average)
Attack:
Kick: 10-20 (10 stm)
His higher stats reflect greater strength and durability than Ruby's mother, but his larger size makes him less agile than his mate.
Let's see what would happen under this proposed system if Ruby's parent's were to try and hold off Redclaw for a round or two of combat.
Redclaw has 3500 HP, so there's no way that Ruby's parents can beat him on their own, and even if they could do enough damage, they would run out of stamina...
Round 1
Ruby's mother kicks Redclaw. Her base agility is 14, and the added number this round is 10, giving her 24. Redclaw's base agility is 8, and he only rolls a 2, giving him 10. Because he failed to dodge the blow (due to a lower agility roll) her kick hits, with 7 damage, but against his armor rating of 12, it fails to do any damage.
Ruby's father kicks Redclaw. His base agility is 12, and the added number this round is 20, giving him 32, essentially guaranteeing a hit. Redclaw's base agility is 8, and he rolls a 10, giving him 18 altogether. Ruby's father hits, with 18 damage. Since Redclaw's armor rating is 12, 12 points are subtracted from the damage of the kick, which causes Redclaw 6 points of damage, leaving him with 3494 hitpoints left.
Redclaw makes a bite attack at Ruby's mother. His base agility is 8, and the added number this round is 12, giving him 20 altogether. Ruby's mother has a base agility of 14, and adds 7 this round, enabling her to barely dodge the attack with a rating of 21. No damage is done to her because Redclaw's jaws missed.
Round 2
Ruby's mother flees the battlefield. Because Redclaw is still fighting Ruby's father, he makes no attempt to persue her. She leaves the battlefield, and in one round, is out of danger.
Ruby's father kicks Redclaw, has 20 agility altogether in this round, and hits for only 2 damage, giving Redclaw a total of 3492 hitpoints left.
Redclaw uses his kick attack on Ruby's father. After adding up the agility stats, they are exactly even! In this case, the blow is a glancing one; half-damage is inflicted after all stats are added up. Ruby's father has an armor rating of 9, the kick does 69 damage; subtracting 9 from 69 gives one 60 damage, but since it was an indirect hit, it only does 30 damage to Ruby's father, giving him 105 hp left.
Round 3
Ruby's father flees. This time, speed is important; since Ruby's father is faster than Redclaw, he can expect to escape with ease, assuming that there are no obstacles in his way, or any other predators (will deal with obstacles later.) Ruby's father has a speed of 16, Redclaw has a speed of 10. Ruby's father escapes.
This combat system I have here is partially taken from various Multi-User Dungeons (MUDS), but I have simplified the system somewhat.
Also keep in mind that some of these stats could be useful outside of combat; a dinosaur who has to travel over volcanic terrain might be able to survive it with a ton of hitpoints (like Redclaw), or might be able to to use his or her agility to avoid being hit to begin with (like Ruby's parents.)
Special Abilities
While the odds of these being implemented are probably slim to none, these might be worth considering.
In order to give certain characters an "individual touch", certain special abilities and moves could be added to them. Alternatively, these could be taught to the player by the character who knows each ability.
Here are just a few examples:
Tops
Special Ability: Unstoppable charge.
Tops is a very strong dinosaur; in order to become the Alpha male that he is, he has had to literally run into the opposition and come out on top many times. Given his years of experience, it is no surprise that he knows how to charge an enemy.
Usable by high-strength, four-legged characters only, this ability can only be used at the start of a battle and once every 5 rounds, at will. Use of this ability causes the attacker to totally ignore armor and to always do the maximum damage allowed for the attack. It uses a great deal of stamina (100), but if it hits, it can be devastating.
If the attack is not used during the next fifth round, it is still available until it is used. A stack of "saved up" charges cannot be used; only one potential charge can be had at any one time.
Tria
Special Ability: Anatomy
While she isn't as strong as her mate, Tria can still do damage in a fight. Placing a high value on the comfort of her body, Tria was always conscious of the effects of stress and pain and learned how to alleviate those things. In the process, she has gained general knowledge of muscles, nerves, and weak points, and knows where to hit in order to cause the most pain and damage.
Usable by any character with sufficient agility, this attack comes into effect whenever a character beats another's agility roll by 10 or more. Half again as much damage is done in the attack (round down if need be), and the effects of armor are halved against for this attack. While rarely used and not usually devastating, it can make the difference in a close battle.
Also, outside of combat, this ability can be used to help heal others or to wake others up by poking them where it hurts.
Doc
Special Ability: Semi-Prehensile Tail
An ability only open to him, Doc has an unusually-set series of joints at the end of his tail, enabling him to grab things in and out of combat.
Out of combat, Doc can use his tail as a biped could use one of his hands to hold a single item. In combat, all Tail attacks Doc makes have an extra + 4 bonus to their agility.
Redclaw
Special Ability: Jumping Bite
Usable only by a handful of the largest predators, this attack has been known to bring down the largest of dinosaurs.
Usable at the start of combat and once every five rounds against a 1000 + starting hitpoint creature only, this attack can cause the most damage of any. For the attack to succeed, Redclaw must beat another creature's agility by 10 or more. Using the attack requires two stages. First, if the agility is beaten sufficiently, it puts Redclaw on the creature's back or close enough to it to bite it in a vital area. A second agility roll must then be made, and it must beat the target's agility by 5 or more. Success will have Redclaw bite for double maximum damage against halved armor (500 damage in one bite minus halved armor!) Only one potential bite may be "stored" at any one time.
Bron
Special Ability: Body slam
Bron epitomizes the Sauropods; big, strong, and heavy. There is nothing tricky about this attack, it simply deals a lot of damage, as a creature weighing many tons throws himself into his opponent.
Usable once every five rounds and at the start of combat, this attack does 350 points of damage. In order for it to hit, the agility roll Bron has must be at least 10 more than the target.
Sue
Special Ability: Crush
For all her kindness towards Littlefoot and his grandparents, Sue can have an arrogant streak. Claiming that her big Sauropod feet give her the right to step on creatures she deems villainous, Sue has a well-disguised "might-makes-right" mentality, at least, when it comes to combat.
Usable only against creatures with fewer than 1000 starting hitpoints, this attack can only be used once every three rounds. For it to hit, Sue must beat the opponents' agility by at least 10, upon doing this, she stomps on the opponent. The opponent takes between 30-45 damage (minus armor), and is unable to move or attack for the rest of the round. The next round, the creature must beat Sue's agility by 10 or more in order to get out from under her foot. If the creature escapes, it takes no damage that turn, and neither Sue nor the creature can attack that turn. If still pinned, the creature takes another 30-45 damage (minus armor) and cannot move or attack. This continues until the creature escapes, is dead, or Sue decides to stop the attack. If fighting multiple opponents, Sue can make attacks on other enemies that are near her, but she cannot move away from her position while maintaining the Crush.
Ducky's Mother
Special Ability: Evasion
Duck-Billed dinosaurs like Ducky's family are not designed for combat. Rather than fight, these peaceful herbivores would rather run to the safety of the water and swim away. In times of battle, it is best for them to avoid attack.
Use of this ability is possible once every three rounds, and it adds an extra + 5 agility against being attacked at the cost of -5 agility to attacking others. Furthermore, Evasion also enables an immediate exit from combat if fighting next to water.
Outside of combat, this can be used to avoid moving dangers, like falling boulders, or lava.