You could use the
mobs list on the wiki to get an overview (a little outdated of course) of what mod contains which animals.
Although you are probably rather suggesting a centralized category system of sorts for "official" mods that the modders have to stick to. Sorry but Minetest is not a game that has a set single direction and a single captain that is in command. Minetest is mantained by
that do whatever they think interesting and of those that do some steering in some general direction not all are always on board let alone at the rudder at all times.
So any modder can provide any mod he wants to (license restrictions put aside for a moment) containing any content he thinks fit and any user can download whatever he is interested in. The problem is there are way over 700 mods by now and it may be a little difficult for a new user to find just the right mod he is looking for.
A little help may be the "
Search Minetest mods" page that offers a somewhat better searching tool than the inbuilt search function on the forum. Not to forget the "
Minetest Mod Database" that may one day, when all problems are solved, work as intended.
You may not be aware but a lot of "mods" are in fact modpacks such as homedecor, technic, mesecons, plants_lib or 3D-armor. Besides that the subgames are essentially just tweaked collections of mods collected to achieve some sort of themed gameplay. You may want to study the "
List of Subgames" at the wiki for a larger though always incomplete overview.