All of the currently confirmed dinosaurs and creatures in the game and some that the devs have stated plans for adding. Dossier can be clicked to via the dossier for each creature. This list isn't necessarily 100% accurate but should be a good representation.

R=Ridable
T=Tameable
U=Unknown
N=Neither


Hadrosaurs (confirmed) (R/T)
Brontosaurs (Ingame) Dossier (R/T)
Stegasaurs (Ingame) Dossier (R/T)
Trex (Ingame) (R/T)
Utahraptors (Ingame) Dossier (R/T)
Triceratops (Ingame) (R/T)
Anklyosaurs (Ingame) Dossier (R/T)
Parasaurolophus (Ingame) (R/T)
Spinosaurus (Confirmed) (U/T)
Pachycephlasaurus (Confirmed) (U/T)
Iguanadons (Confirmed) (R/T)
Carnotaurus (Ingame) Dossier (R/T)
Dilophosaurus (Ingame) Dossier (T)
Gallimimus (Half Confirmed) (U)
Compsognathus (Confirmed) (U)

Titanoboa (Ingame) Dossier (N)
Plesiosaur (Ingame) (T)
Mososaurus (Confirmed) (U)
Sarcosuchus (Ingame) Dossier (R/T)
Pteranadons (Ingame) Dossier (R/T)
Dimetrodon (Confirmed) (U)
Carbonemys (Ingame) Dossier (R/T)
Liopleurodon (Confirmed) (U)
Ichthyosaurus (Confirmed) (U)

Beezlebufo (Confirmed) (U)

Giant scorpians (Ingame) Dossier (R/T)
Giant land and sea snails (Confirmed) (U)
Giant ants (Confirmed) (U)
Giant dragonflies (Confirmed) Dossier (U)
Giant spiders (Ingame) Dossier (T)
Very Giant Spiders (Ingame) (U/T)
Giant crabs (Confirmed) (U)
Eurypterid (Confirmed) (U)
(possibly more)

Mammoths (Ingame) Dossier (R/T)
Saber tooth tigers (Ingame) Dossier (R/T)
Ground Sloths (Confirmed) (U)
Gorgonopsid (Confirmed) (R/T)
Giant Bats (Ingame) Dossier (T)
Phiomia (Ingame) Dossier (R/T)
Gigantopithecus (Confirmed) (U)
Mesopithecus (Confirmed) Dossier (T)

Dodo (Ingame) Dossier (T)
Argentavis (Ingame) Dossier (R/T)

Megalodon (Ingame) Dossier (R/T)
Coelocanths (Ingame) Dossier (N)
Dunkleosteus (Confirmed) (U)
Tusoteuthis (Confirmed) (U)
Mega Piranha (Ingame) Dossier (T)


Dragons (Ingame) (N)

This information borrowed primarily from http://gtm.steamproxy.vip/app/346110/discussions/0/613957600548030130/