I suppose it may well be taken as a hint that Dinah and Dana are not exactly the same species as Cera
absolutely possible, but well, I of course compared the bodies, too and, disregarding the size, they're similar, what makes me doubt them being of different species. If it really is like this, why making the only difference between two different threehorn tribes being two additional horns, which in fact are small and likely to be overseen?
furthermore in The Wisdom of Friends Cera says that she will grow two more horns.
Now i will be brave and do an interpretation of the creator's acts.
They might know that a triceratops has three horns, therefor Cera's state in movie 13.
But thinking of the first, maybe they just found no way to do both, the main chara expression thing and the three horns. They decided to do the thing they regarded as more important to the movie itself and left out two horns. To say, it was kind of a technical problem. And even if they had found a way to do both AFTER the movie of 1988, just adding the two missing horns to Cera's forehead seems a very hard change of the chara, in my opinion, even if it would just be her outlook. But still being aware of the problem, they finally did that particular state in movie 13, which was definitely not necessary to the story itself.
Of course she doesn't need to care about things like that at all.