It punches holes in boulders on stream-beds. This is a clever innovation that prevents its eggs from being attacked or washed away by the current.
(From Bulbapedia) Seaking is a large, orange, fish-like Pokémon. It has billowing caudal and pectoral fins, all of which are white specked with black. However, its dorsal fin is pure white and rigid. It has round, dark eyes, prominent pink lips, small fangs, and a horn in the center of its forehead. Females have smaller horns than males.
Seaking is a very powerful swimmer that is found in rivers and streams. It can not only swim against a river's current but it can travel up waterfalls. In the autumn spawning season, it travels upriver, where male Seaking, grown fattier and more vibrant in color, engage in elaborate dances to court females. Seaking uses its powerful horn to bore holes in riverbed boulders in order to shelter its Eggs from water currents. A mated pair takes turns patrolling around their new nest for roughly a month, and defends it with their lives.
(From Bulbapedia) It seems to be based on a species of goldfish known as Azuma Nishiki (東錦), bearing a similar coloration to shubunkin goldfish. The horn part may be based on Matsya, an avatar that the Hindu god Vishnu assumed, which is sometimes depicted as a fish with a horn.
(From Bulbapedia) Seaking is a combination of sea and king. Additionally, it may be a pun on "sinking".