Its a question of "combat sports". Sure there are tons of super strong animals....however the real question to ask is: How many of these animals regularly fight to the death?
A gorilla is super strong, but fatal fights are exceedingly rare. Same with most other animals.
A new male lion coming into a territory will kill the existing male and all the cubs in order to gain access to the females, thus this presents a "do or die" proposition that they are willing to fight to the death over this.
Wouldn't surprise me if it's some sort of crab. Near-inpenetrable shell, massive claws, once you remove the size advantage of other animals then the crab becomes quite viable.