Book about a high school kid raising a miniature horse

[pmueller78]

I'm looking for a book I read probably in the late '60s or early '70s. It was about a high school kid who was raising a miniature horse to enter in some kind of interplanetary competition -- sort of like FFA of the future. I think he had a friend who was doing the same, but the friend's horse got sick or died before the competition. Somehow the first kid came into possession of a rock-like object which turned out to contain an intelligent alien that resembled a turtle with a long neck. Does this ring a bell with anyone?