I admit, this villain is the hardest to believe.  I suppose it is possible like they did with the Carjacker appearing to be the killer of Uncle Ben in Spider Man but Flint Marko turning out to be the actual killer in Spider Man 3.

It would seem that this works best if he appears in no scenes where he'd be directly in a scene from the original film.  The premise behind him is that he is the real villain of LBT, sort of the ultra villain, Littlefoot's archenemy, and Sharptooth was only an accomplice in what happened in the original movie.  

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Gallim was a Bothtooth.   Gallim actually admired Sharpteeth, though he thought them inferior.  While most Bothteeth would probably only eat clams and things like that, if they ate meat at all, Gallim was different.  The thrill of the chase.  Watching your prey die, it amused him.   He loved the taste of blood.  He even liked Sweet Bubbles so much because it reminded him of blood.  Yes, he was a Sharptooth inside, in the darkest sense.  

"We are special son." Gallim's father told him.  "Why?" the young Gallimimus asked.  "You see, we are neither Sharpteeth nor Flatteeth."  said his father.  "Yes, I've noticed.  We're both."  said Gallim.  "Precisely." said his father.  "We can survive the changes that are affecting the rest of the world.  Our kind can switch diets while the others will perish.  Perhaps we can come in time to help the others, but it seems that fate has decreed that our kind should be the ones in charge.  Even if the others die out, we stand the best chance of staying around.  As such, we  need to make sure that we have as much power over the other kinds as possible so that, if the others don't hold up, we can take over." said his father.

Gallim liked the idea.  However, right before he came of age, his father and mother, involved in a local feud, had perished.  Gallim had avenged their deaths, of course.  However, there was still their plan.  It looked like he would have to take it upon himself to bring in the New Bothtooth World Order.  

He would need Sharpteeth to begin his conquest.  Flatteeth henchmen, at least to start out, wouldn't do.  He would start by practicing misdirecting leaf eaters by using his leaf eater speaking ability.  This would help him for later when he would take over many places, including, he hoped someday, the Great Valley itself.   He was of the special kind after all and could speak both Sharptooth and Flattooth.  

He found a very promising accomplice.  One simply known as "Sharptooth" by the local leaf eaters.  Gallim was wise enough to try and talk to Sharptooth from a distance, not wanting to become a meal.  Soon, his idea of misdirecting leaf eaters got Sharptooth's attention.  Sharptooth was certainly brutal and could bring down leaf eaters, but it wasn't easy and easy work seemed to please him if he didn't have to fight so many leaf eaters at once.  

Gallim had learned that Sharptooth also had a brother named Red Claw.   However, he didn't know where Red Claw was.  Gallim hoped to meet Red Claw soon as two fearsome Sharpteeth would speed up his plans.  However, he needed to work with the partner he had.  Besides, getting too many Sharpteeth when he wasn't quite ready to handle that many was a sure way to be made dinner if he wasn't careful.  

They began one day around dusk, wandering around.  The two had agreed to split their takings half and half.  Sharptooth, who was bigger and stronger, well, somewhat stronger, as Gallim had quite a bit more strength than he let on,  was upset that he wouldn't get more.  However, Gallim reminded him that he was the brains of the operation.  Sharptooth had given in.

Gallim told Sharptooth to go and hide and roar when he would signal to him.  He went send the leaf eaters off and they'd go away from the main herd, which would be going off away from, as the believed, Sharptooth.  Gallim and Sharptooth would double back away from them and go and eat any stragglers.  

Gallim came to a clump of trees where Club Tails, Longnecks, and some others were there.  They kept apart due to herds always being separated.  Gallim wanted to blend in and so get them to focus on him so as to get the Leaf Eaters from accidentally stumbling upon Sharptooth and giving the trick away.

He went and ate sweet bubbles, which he liked.  A Club tail told him to go away, that he wasn't a club tail and so wasn't welcome there.  Gallim didn't mind and said he understood herd segregation and said that it was normal.  

He next went near some longnecks.  However, Grandma and Grandpa Longneck didn't appear to mind him.   He found this odd, as herd segregation seemed to rule supreme out here.  The Club Tail told the longnecks to tell Gallim to go away.  However, they refused.  

Gallim, having gotten the attention of everyone enough, signaled Sharptooth behind his back, out of the eyes of the others.  Sharptooth roared, as they had agreed.  

Gallim said that he thought that he had seen Sharptooth nearby and was heading a certain way, and had thought that he'd lost him.  He sent them away from Sharptooth.  Most took off to save themselves.  Gallim had Sharptooth move out of their way in time and he was hidden.  The plan was working.  

However, the longnecks stayed to make sure he was all right.  He assured them he was.  They told him they were glad that Sharptooth had gone the other way, for their daughter Arianna and grandson Littlefoot had no idea of the danger of Sharptooth.  The two longnecks then left.

Sharptooth came out.  Gallim told Sharptooth that the herd segregation made life so easy for them.  He said that if the Leaf Eaters were to ever learn to work together,  the two of them would be in hot water.   Sharptooth said that sadly Sharpteeth suffered from herd segregation as well.  

Gallim said nothing. If there was one thing he feared more than Leaf Eaters working together, it was Sharpteeth working together.  No, best to leave things the way they were.   The other kinds were as strong as they were united and as weak as they were divided.  Gallim wished to keep them really divided.  

Gallim knew where the Longnecks had come from.  As he knew they were tolerant and, as such, dangerous to his new world order, they must die now before they could be a threat later.  Gallim grinned at Sharptooth and said "I've decided.  We're having longneck steak tonight."  The two laughed maliciously.  

They came near the sleeping longnecks, who were with their daughter.  The grandson seemed to be missing.   Gallim was going to kill the male one first, but, before he could get too near, he turned and saw that his partner had gone off