“Oh! You trained him to stand on his hind legs?”

While walking around your neighborhood, you see your neighbor, who's playing with her dog. The dog is standing up like a person. You're delighted to see that, so you say:

Oh! You trained him to stand on his hind legs?

Oh

This is a sound that people make when they've learned something that surprises them.

Oh, you're not coming? I thought you were.

A: Excuse me, miss. We're closing in five minutes.

B: Oh, OK.

Oh, I guess we're both going the same way, huh?

train (an animal) to (do something)

You "teach" people, but you "train" an animal. 

You can sometimes use the word "train" for a person, but it suggests training for fighting or the military. And it's also common to joke about women "training" their husbands or boyfriends:

I had to train him not to leave the toilet seat up.

(an animal's) hind legs

Animals that walk on four legs, like dogs, have "front legs" and "hind legs".