The way that meshes show the role and use of an animal in the gameplay and the game world

Thinking about this topic, the first thing that comes to my mind is the level of detail. An animal that would be seen from a closer viewpoint and for a longer period of time on a player's screen should be way more detailed than one that is just running around in the background. The best example that I can give is the birds in games. For the ones that are seen from further away, the geometry and textures are very simple, considering that there is no way the player would see them from up close. On the other spectrum, a bird like the eagle in Assasin's Creed would require a lot more attention to detail, being treated like any other character.

This applies in regards to the rigging and skinning as well, animals that are seen in the background having a very simple bone system, while animals that are way more into the focus having more complex ones, with additional bones depending on the case. 

Topology is something that also comes to my mind while thinking about this, an animal that would be shown in a cutscene requires some nice topology on the face for the facial animation, while an animal that would be seen walking in the world would not need something like that.