Is it possible to create custom unit skins? Are there any mods?

Hey, I'm a graphic designer who would like to try creating custom unit skins, maybe something like a knight for non-european nations that doesn't look like an european knight. It feels weird to see Chinese faction with those European-looking cavaliers. I know that some heroes can be substituted (I've got modded paladins with lances), but I couldn't find any custom skins or tutorials how to make and implement them. Are there any sources?