Anything with a model extending ModelBiped should be able to sit down on horses, rather than standing on the saddle. I haven't messed with rendering too much, so idk for sure how this would be done.

Well, I added riding pose support for the actors in Blockbuster mod recently, but didn't quite get the change to update the custom model code in Metamorph mod. However, I think it looks pretty funny when the player stands on a horse or pig while morphed into silver fish or something 😂

Patched in 4f60335 for any mobs that support riding. Unfortunately, pigs don't ride 😞