According to Petkeen, both words are Spanish, and “grulla” should be pronounced as “grew-ya.” This obviously isn’t the phonetic pronunciation, but knowing how to say it correctly will avoid a lot of confusion, and will ensure you don’t embarrass yourself around breeders. “Grullo” is also pronounced unintuitively, “grew-yo,” so make sure you learn both pronunciations, especially if you intend to keep these kinds of horses!