Wrestling Trolls: Big Rock and the Masked Avenger by Jim Eldridge

Wrestling Trolls Book One
 
Jack knows every wrestling move ever, and all his dreams come true the day he's recruited as a training assistant to Waldo's Wrestling Trolls. But little does Jack know he's actually part troll, and when his new family is in trouble he morphs into the terrifying Thud!
 
With Milo, Big Rock and Robin the grumpy talking horse, the friends find themselves rescuing princesses that don't need rescucing, breaking out of dungeon prisons and taking on Wrestling Orcs in a battle for the tournament prize money. 
 
Suitable for ages 10+