I think that Ozzy and Sierra are psychopaths. Nothing else explains their bizarre and murderous behavior.
Sharptooth might be a bit out there too, as you can see him snapping at thin air, even though he's got to know he can't get at the Gang of Five. Also, he seems so willing to get the kids that he goes into a thorn patch (even the hyenas from Lion King knew a thorn patch was a bad idea and got out quickly.)
Pterano I don't think is nuts, just thinks too highly of himself.
Spike, I used to think had some kind of mental disorder, and that was why he almost never talked. However, after seeing "Through the Eyes of a Spiketail", I can't conclude that anymore, though I can conclude that he's color blind.
Topps's behavior is more of a mystery, though perhaps, like the Gang of Five, he was bullied by some Hyp types as a kid, and eventually was taught to stand up for himself by Grandpa Threehorn (Topps's father), and is now taking the standing up thing too far. As Topps actually says that his father made him what he is today, we can surmise that Grandpa Threehorn is just as bad, if not worse, than his son, including possibly in species bigotry.
Cera might be narcissistic, on a lower level, as she seems stuck up, and, even at the end of the first film, never admits she was wrong for leading the others the wrong way (heck, Pterano got banished for such an offense! The only difference between his herd and Ducky, Petrie, and Spike is that the latter 3 lived, though she had kept going and they could easily have died in the first film.) and Ruby says in the TV Series that talking about Cera is one of Cera's favorite things to talk about.
Petrie might be somewhat panicky or insecure as, of the Gang of Five, and, indeed, Chomper and Ruby, he is the only one that seems to lack a father figure in his life.
Ducky's Father might be really shy as he never seems to talk ever despite having made it to the Great Valley (though perhaps this might have something to do with what happened to Judith)
Ducky actually seems quite stable, and, in fact, rarely gets mad. On the other hand, she was really odd in the Sky Colored Stones episode where she was ignoring Spike when he was stuck and hoarding those stones kinda like Gollum and it doesn't take much to see her going "My precious! Don't worry my precious! Ruby and Cera won't get you!" In fact, she seems to have trouble with that last stone, wanting to hold onto it even though her life was in danger (Chomper behaved similarly with his tooth that fell out, but, to his credit, he thought he had to get it back in or he'd fall apart into bones. That wasn't the case with Ducky. Yeah, Littlefoot was like that with the Tree Star his mother gave him, but that's more understandable and, also, he leaves it to save himself and the others when they run from Sharptooth.)
As for Bron, i personally believe that he ran from Sharptooth or something and got away, but hit his head in the process and got Amnesia and forgot his family for years. Also, this chased by a Sharptooth scenario makes it plausible why Littlefoot's mother or Littlefoot's grandparents never mentioned him to Littlefoot in the films. They would all think he had died, which explains why they never went looking for him.
I know, Grandma and Grandpa Longneck don't appear too shocked to see Bron still alive, but the Bron scenario seems it's gonna have problems somewhere no matter what they add to it later on.
Perhaps they thought Bron might have died but weren't sure, and assumed his long absence meant he had either left or died. They never told Littlefoot as they didn't truly know what had happened to him.
I did create a villain named Gallim, who is quite manipulative, and also chameleon like, being an omnivore, he can pose as both a Sharptooth and a Flattooth and secretly hates both kinds, though he often works more with Sharpteeth as they are more brutal and more to his liking. However, brutality isn't his greatest weapon, but his mind is. He especially likes to keep herd segregation around forever as he knows that if they were to ever work together, they could pose too great a threat to his new world order plots.
He can gloat over horrible tragedies and has no pity on anyone. Near the end of the two part finale film, like Claude Frollo, is starting to lose it and is cackling madly and, like Frollo, is saying that he wishes he'd take out Littlefoot 20 years ago and how Littlefoot's mother died trying to save Littlefoot from him (Sharptooth turns out to be Gallim's past accomplice, with Red Claw, who turns out to be Sharptooth's brother, being his current Sharptooth accomplice.) and now Littlefoot is trying to save Ali. He vows to slowly kill Ali in front of Littlefoot just to torture Littlefoot and is seen getting more maniacal as the final fight progresses.
Gallim even has other creeps (Sharptooth, Sierra, Red Claw, and Ozzy) working for him at various points.
As for asperger's, I have that too.