there are animals that can't be added to the tribe
but you can add any wild animal to te tribe whose tamed version is already available. you can make the dustriders have a wild mammoth e.g.
the easiest way is to use the cheats (Link viewtopic.php?f=87&t=1130
) and to modify the hero spawns to those of the units you want. for example you can replace the line cole_s0 at the very top with the line brachiosaurus_s0 for a brachiosaurus, and replace the line bela_s0 with the line atroxosaurus_s0 for a wild trex. i'm afraid diplodocus or spinosaurus make the game crash (?)