Mo' creatures got a lot of horses and horse like creatures that you can ride. would be really cool if this mod would be compatible with those.

version 1.20.1
Forge

They don't extend vanilla AbstractHorse, so most Horseman features would need to be written specifically for their entities. It wouldn't be easy to add a support for it. I think the better way to make such compatibility would be to have a separate mod for it (with dependency on both Mo' Creatures and Horseman), where Horseman's features could be reimplemented for Mo'Creatures horses, reusing Horseman's code where possible.

I don't have the time and desire to do it, so unless someone else would decide to write such a mod, it'll not be done.