A lunk was a large type of animal lifeform, a mollusk native to the oceans of the planet Sanctuary.
History and specifics
In the year 2268, Spock was trying to swim to the shore of Khyming from a sailboat when he was attacked by Old Hemcree, a renowned and ancient lunk. Spock was able to kick away from the giant creature, but just barely. (TOS novel: Sanctuary)