Bucky knows that as an omega, he'll be expected to act like the transformation is every bit as painful as it actually is, but he can't let himself show any weakness. He's the prince, he won't let himself show any weakness. He stops himself from yelping, but only barely-- still clinging to Steve's hand.

He can feel the change come on rapidly, elongating his limps, changing nails into claws, tail growing out from under his ceremonial robes. It's enough to force himself to pull away, lest he accidentally claw his almost-mate from the pain. He collapses on his knees, growling and whining, digging his claws into the ground.

He's not so sure his throat will work right now, he can't cry out to Steve, but he watches his mate anyway, a possessive part of himself ready to attack anyone who comes near him, to make it clear this is his alpha.