Let's make our own models (and maybe even animations)

Hmm, not ideal. We copy the entire file system with the desired creature into the map archive. We simply remember (or write down) the paths to all the necessary files (e.g., .(Character), .(Model), .(AnimSet), etc.), and then, as usual, create a hero with the model of that creature (like in my manual). I used a similar approach with the Kraken from Zakhar's mod. The obvious downside is the huge map size (+20 MB).