Talk:Animals/@comment-5092634-20181007040638/@comment-24468230-20181007050540

I think it could be based on other hooved species being "similar" to horses.

In our world, certainly closely-related animal species can breed, like lions and tigers. In Equestria's world, maybe a similar rule applies to related species being able to understand each others' speech. (Certain ponies like Fluttershy, who have animal communication as a talent, are exempt from the rule.)