First of all, the new models for the Tyrannosaurus and Gigantopithecus look really good. The dire wolf could use thicker fur on its legs. Otherwise, it looks much better. There are several other animals I also feel could use a rework. These changes could accompany appropriate changes to the dossiers pictures too.
Triceratops: For the love of God, please change its leg movements while its running. A real Trike wouldn't run like that. Make it and the Pachyrhinosaurus run more like the Morellatops. The Trike could also use a serious size increase (like 20-35%). I mean, it looks puny compared to the Tyrannosaurus or even other herbivores like the Stegosaurus. Updating and changing the skin textures wouldn't hurt either and maybe give it some interesting patterns on the frill. Also, buff its overall stats. A Trike should be a much higher tier mount and should be able to at least give a Rex a hard fight.
Stegosaurus: Mostly cosmetic with this one. Shorten the neck just a little, update the skin textures, increase the size of the back plates a little. Maybe give it a spike impale attack like the Kentrosaurus.
Ankylosaurus: Similar to Stego. Update the skin and armor textures, increase the size by about 30 or so percent. Maybe give it a club attack that can instantly break bones of wild and enemy creatures.
Brontosaurus: Update skin textures, make the neck curve under rather than over. Perhaps give it the ability to trip large carnivores with its tail whip.
Parasaurolophus: Increase in size by 30 or so percent, update skin textures, increase the skin flap between neck and crest, give it a tail swipe attack with knockback, make it functionally bipedal/quadrupedal like the Iguanodon, give it an alarm system ability to alert you of hostile creatures in the area, and since in the dossier it says it is somewhat amphibious, maybe give it infinite oxygen while swimming.
Utahraptor: Give these poor raptors a full body of feathers, confound it! They look naked and stupid without feathers. If that can't be done (which I doubt, but still), perhaps increase the detail and amount of feathers on the places they have them currently. Reshape the head to be a little more like in the dossier too (i.e. less blunt and slightly more slender of a snout).
Woolly Mammoth: Retexture the fur and maybe give it a trumpeting ability that can rally other allied mammoths to its aid. Maybe a herd buff since mammoths, like elephants in real life, lived in large herds.
Woolly Rhinoceros: Retexture fur and skin on face and feet. Maybe retexture the horns, eyes, and toenails as well.
Compsognathus: Fully feather (if possible) and retexture. Maybe give them a gnaw attack where they latch on to a creature/person and slowly drain their health by biting them continuously (kind of like in The Lost World: Jurassic Park). Similar effect and removal process to a leech or lamprey.
Carbonemys: Retexture shell and scales. Maybe a body slam attack.
Titanoboa: I don't even think I need to say this, but I'm going to anyway. This things looks puny compared to the new basilisk. Increase the size and give it a constriction attack.
Pumonoscorpius, Araneomorphus, Dung Beetle, Arthropleura, Titanomyrma: Retexture exoskeletons. Give the Pulmonoscorpius the ability to grab players and small creatures with its claws and slowly apply torpor (similarly to Tusoteuthis's tentacle grab). Give the Araneomorphus the ability to create web traps for small creatures. Arthropleura should probably have the ability crawl up walls (as a matter of fact, this and the other two should have this ability). Titanomyrma should have a tamable queen variant like the giant bee.
Dilophosaurus: Increase in size by about 50 or so percent, give it a pack boost capability too.
Spinosaurus: Make this one functionally bipedal/quadrupedal so it can attack with its hand claws, update skin textures and swimming animations, increase size so it is bigger than the Tyrannosaurus.
Mosasaurus: Make it look like it actually has scales, reshape head a little bit (i.e. sculpt it down a little).
Elasmosaurus: Change the head to what it would really look like and make it look scaly. Maybe a mode giving more fine control of the neck for attacks.
Megalodon: Increase size to be about roughly the size of the Elasmosaurus. Give ti a bleeding bite attack.
Ichthyosaurus: Retexture skin. Maybe a spinning attack like the Pteranodon, but for underwater. Maybe a three second cooldown on it like the Ptera.
Argentavis: Rextexture the feathers and the face. Give it a divebomb ability against players and small creatures.
Beelzebufo: Smooth out the skin. Maybe a passive poison that induce torpor when physical attacks (creature melee, player fists, etc.) touch it.
Carnotaurus: Slight retexture. Maybe an attack with a charge up similar to the woolly rhino's (and eventually Trike's) skewer attack.
Castoroides: Retexture fur. Maybe allow it to build dams that act as primitive defense walls that can be built in shallow water and on land (would require so much wood and thatch per dam in inventory). This would be sort of like how you create lassos on the Equus.
Dimetrodon: Slight retexture. Maybe make it faster and less sluggish, able to amply keep up with a player and hunt down small creatures (would make it more threatening).
Dimorphodon: A little bit more fur/pelage would go a long way. Maybe a flock bonus.
Diplodocus: Increase in size and length by about 50%. Give it attacks that actually do damage (maybe not in the wild, but when tamed most certainly, e.g. tail whip, stomp, etc.).
Dire Bear: Pop the snout out a little so it doesn't look like some kind of teddy bear, maybe give it the ability to sit like the Chalicotherium and Megatherium.
Dunkleosteus: Update textures a little, give it a bleeding chomp attack.
Gallimimus: Fully feather it up. Give it a kick attack maybe.
Giganotosaurus: Shrink by about 40% and retexture a little. Increase the diversity of the color palette without breeding mutations too.
Kairuku: Retexture feathers, give it a hunt and retrieve fish capability similar to Ichthyornis.
Kaprosuchus: Retexture scales to make it look more unique from Sarco and increase color palette. Maybe a gash attack like an alpha Allosaurus or Ravager.
Manta: Maybe a retexture and a speed boost move.
Megaloceros: Make it look more like a real elk. Make the fur more detailed. Give both genders a kick attack like the Equus.
Mesopithecus: Update the face and fur like the Gigantopithecus. Give it a lost item retrieval ability (say if you threw away something you wanted to keep by accident).
Onychonycteris: Remodel to make it more like a real bat. Maybe have them roost on cave ceilings and walls. Maybe a group bonus as suggested for the Dimorph.
Oviraptor: Fully feather it. Maybe give it more of a role in baby creature care (feeding babies and such) so that it isn't just dirty egg thief.
Pachy: Increase the size by about 75%. Retexture and update model. Maybe give it the ability to harvest large amounts of flint or crystal with its headbutt.
Paraceratherium: Retexture the skin to make it a little more rhino-like. Maybe a herd bonus like the mammoth.
Phiomia: Update skin textures. Maybe reduce weight of berries when harvesting with it.
Megapiranha: Remodel to make them look like real piranhas. Maybe a pack bonus for increased damage.
Procoptodon: Make the fur look smoother.
Pteranodon: Increase muscle mass, retexture skin, put some pelage on it, increase overall size by about 30%.
Quetzalcoatlus: Increase pelage amount. Remodel it so that it stands more vertically like in the dossier. Maybe a beak jab attack that deals piercing damage.
Sarcosuchus: Retexture skin, give it a death roll attack, and the ability to carry around creatures like the Kapro. Remodel so the jaws can actually close properly.
Terror Bird: Make it taller and retexture the feathers. Give it a beak hack attack to slow down enemies and creatures, which would also deal piercing or bleeding damage.
I know it's a long list, but I think most of these are purely cosmetic. Just my thoughts.