Trike: Updated animations, along with a new ability and model. I was thinking it could have a "herbivore's buff" when up against large predators, giving it slightly increased resistance and melee.
Stegosaurus: Like the Trike (rhyme baby), the Stegosaurus is found to be obsolete even in the early game (thanks to other creatures that do its job faster). A new ability/enhanced stats could help the Stegosaurus be effective until the even late game. A model upgrade would be very nice, and I'm curious to see how the Devs would model a Stegosaurus after all their experience. I hope to see staple-childhood creatures like this, and the Trike, be made useful for the whole of the game.
Utahraptor: Not very useful, outdated model, weak; this creature may be in one of the worst states in the game. Give it a pack-buff, give it a grapple for large prey, and update that model dammit! It would be very cool to see a feathered model for our old friend, and an increased health pool never hurt anybody. A buff to melee, better animations, and a heightened jump are my last suggestions.
Compsognathus: Our tiny Compy companions are pretty much useless in all regards, with one idea for an ability to change everything. They currently have a pack-buff, but what if they had a unique pack-buff? Look, a pack-buff that grants them the "swarm" ability, in which they CLIMB their quarry with the intent of shredding them up! The current pack buff would stay, with the "swarm" ability only being activated after your pack meets or exceeds 10 Compsognathus' (the more you have, the more damage "swarm" does and the longer it continues). I also have a bonus idea, which goes to all the smaller creatures (around Hyeanodon size and below). Make some (or many) attacks towards smaller animals miss as if they had just dodged the attack. This would create an incentive to tame certain creatures for more than just being bite-sized cuties. PS, updated Compsognathus model, please! Carnotaurus: Being outclassed by creatures like Allosaurus, Tyrannosaurus, Thylacoleo, etc, poor Carnotaurus needs some love. First of all, a new model. Carnotaurus deserves a model change more than any other (as it's a great dinosaur). Give it some osteoderm hide, more pronounced arms, etc. Give it a roar, more colours, and new animations (running, biting, etc). A new ability for the Carnotaurus could be a "grab", in which it immobilizes prey up to a certain size at the cost of stamina, allowing its allies to deal damage. If that sounds too OP, another ability could be a "charged knockback", in which the Carnotaurus steps back, and headbutts which tremendous force to deal large amounts of knockback (super-effective on small creatures and humans). Brontosaurus: Slow, cumbersome, Brontoboy needs love. I'd suggest a new model right off the bat. Also, a stomp attack would be nice. These mixed with slightly increased movement speed and new animations would suffice. Sarcosuchus: To finish my list, the creature in the most need of TLC. The Sarcosuchus is obsolete in all aspects. First off, an updated model could potentially make the Sarcosuchus on of the most terrifying looking creatures in the game. Secondly, better animations and handling when riding. Thirdly, increased melee, health, and an added bullet resistance thanks to its hide. Finally, give the Sarcosuchus an ability, the dreaded, crocodilian "Death-roll", which involves prey up to Trike size being grabbed (in the water) and twisted around, causing the bleed effect. This ability would be made more powerful as melee is increased, and costs lots of stamina. Also, another passive ability could be immunity to Leeches, as it has become adapted to its Marshy environment (and the hide is too thicccc for dem' Leeches!).
Miscellaneous: Give all mounts some sort of "roar", and rework the kibble system.