Guys, the collar can be resisted, but it's like other spells - it's a spell mastery vs spell resistance check.
If your sovereign is brilliant, wears some mage gear, and has a few levels under his belt, the chance of resisting is not that great. If you are like me, and hoard the collar until you have a worthy target, you probably reach spell master 100, and have a only 3% chance of failure against most beasts.
That said, the most worthy beast of them all is the great wolf. That howl is a game breaker, once you level the wolf.