what's going on with mr. dean's little horse?

Dean is supposed to be riding a small-stature Tibetan palfrey. Because the entire movie was filmed in England, they didn't have the real thing, so they substituted a smaller Shetland pony instead. This was one of the many things that Rumer Godden, the author of the original novel, objected to about the movie. In fact, after seeing the first screening of this movie, she vowed never to let another one of her books be made into a film. She changed her mind about letting them try to make movies from her books, but she never changed her mind about this film.

Mr. Dean is supposed to be the embodiment of worldly desire and sexuality. He wears those tiny shorts so you can see his gorgeous gams. It's supposed to be hot.

He's the answer to the musical question "Who wears short shorts?"

The 'little horse'meant to be a Tibetan kind of pony, is completely in keeping with what you'd find there. His little pants do look wierd, but nothing out of the ordinary for the time... (and may I add that we now have a generation of guys wearing shorts falling off their behinds and hanging down below their knees... which is really idiotic... can't wait for years to pass and they all figure it out). I don't see Dean as asexual at all... quite the opposite statement is being made. The sexual chemistry that exists between he and Sister
Clodagh is a major theme in the film. He's basically 'offered'the Jean Simmon's character too, but I don't think he turns her down because he's not interested... but because he's more interested in doing the right thing with this minor (he's a-religious, but completely moral... another point of tension in the story). As for Sister Ruth... he's sane... she's not... he knows this, and again, does the right thing by sending her back to the convent.

Not really - it's a Shetland pony. They didn't bring authentic horses to the UK just for this film, and it was shot (entirely, I believe) in the UK. The fact that Mr. Dean's feet practically drag on the ground while riding his pony is a reason they shouldn't have bothered using these Shetlands.

One funny thing about Sister Clodagh riding a pony at the end - her pony seems to yawn twice, in a big way, and they kept that scene in, rather than re-shoot it. The pony's behavior distracts from the seriousness of the dialogue.

Except that he implies earlier that he shags a lot of the local women. He doesn't refuse women. He refused one woman, who was a loon, and might conceivably take a pair of scissors to his old fella' while he slept. A wise decision if you ask me.