Half-Man, Half-Bull? Surely you’re joking. But if it’s Dungeon Crawling you need to do there’s nobody better than The Minotaurs.
If a creature is transformed into another creature (say, a human into a dog), then that creature will become more and more accustomed to the new form until a point is reached where reversing the spell is impossible. A second transformation is possible, but the creature will have to go through the whole process again.