Sea Lion

Sea Lions are mentioned in the How to Train Your Dragon Book series.

Description
Sea Lions are mammals adapted to an aquatic life style, but do need to come out of the water for rest and breeding. They are closely related to seals and walruses, but have a few differences. Sea Lions are generally larger and bulkier than fur seals and true seals. They also have ear pinna (ear flaps) whereas true seals do not.

Sea Lions occur worldwide from tropical climates to sub-arctic climates. Interestingly, however, there are no sea lion species in the North Atlantic, meaning historic Vikings would not have much opportunity to come across them.

Function
Sea Lions are mentioned in the Book series as a source for comparison to other creatures.

In reality, Sea Lions - like seas and walruses - were hunted extensively for their hides, meat, and blubber. In modern times, sea lions are also kept captive for zoos and other public events.

How to Train Your Dragon
Sea Lions are mentioned in Book 1 for size comparison to dragons and humans.

"I would say that yelling was highly effective on any dragon smaller than a sea lion."

- Old Wrinkly

"Vast Viking men sat on uncomfortable rocks guffawing loudly like gigantic sea lions in a holiday mood."

- How to Train Your Dragon