Hey Nivex, I think people are having the same issue with invincible horses that they were having with wolves a couple months ago, for the same reason: Facepunch introduced an entirely new horse entity, and shoved the old one aside.
New horses
so i have discovered. just found your post after i asked about this. i can't remember how we fixed the wolf issue.
I think you just change RidableHorse to RidableHorse2