I have no idea what's wrong and was wondering if you could tell me what could be causing the horses to be translucent like this.

What shaders are you using, and which versions? I know Sildur's shaders 1.28 can cause this issue but Sildur's shaders 1.29 works fine.

I have no shaders installed but if that's what causes it. Maybe it's related to a mod I have installed that has to do with shading like betweenlands or disillusioned or something like that.

Dropping support for 1.12.2 - this is probably related to another mod, though.

Dang I just found out today that it was the dynamic stealth mod since I finally had enough time to just try and troubleshoot it. I understand though working on like 3 versions at once sounds like it would be tiring.