A Hammerhead Yak is a type of animal similar to a Hammerhead shark, but shaped more like a yak. They are capable of surviving boiling lava, as one was seen jumping out of a volcano, and later out of the Earth's core. They occasionally appeared to prevent a Boneknapper from killing Gobber. Hammerhead yaks can speak, but they do not speak often. The only line ever said by the hammerhead yak was "deploy the yak." It's first appearance was The Legend of the Boneknapper Dragon along with the Hammerhead Whale.
It's currently unknown if Gobber actually met a real Hammerhead Yak or not, but it is very unlikely.
It is also unknown why they appeared or why Thor summoned them to support a Viking man from a Dragon one-sided.
It is possible that the Hammerhead Yak and the Hammerhead Whale are magical creatures created by Thor. According to Gobber, he did summon them, but there is no information to confirm or deny this theory.
The Yak's atmosphere and behavior is actually similar to that of Po the Panda from the Kung Fu Panda franchise.