The Berserkers are a band of pirates lead by Captain Hammerhand.
They were a very odd band of pirates who first appeared pillaging the northern shores of the Unicorn Forest (in the episode The Terror of Hammerhand ). They have a habit of repeating certain words or phrases three times.
The original Berserkers were apparently all killed when their ship was sunk, but Hammerhand was later mystically resurrected by Mumm-Ra in Spitting Image, who summoned up his spirit to animate a clone of Panthro which he had created. When the plan failed, Hammerhand's spirit broke Mumm-Ra's control, and the clone body shifted into Hammerhand's original form before departing.
When he next appeared, in the Season 2 five-part movie, ThunderCats Ho! The Movie, Hammerhand had reformed the Berserkers with new members endowed with cybernetic bodies and powers like his own.