In the episode Gone Wishing, K'nuckles is seen running from a mob of angry mermen as he had just stolen the The mermiad Queen's heart. He reaches Bubbie in safety and the mermen give up.The same mermen appear in another episode were they teach flapjack and knuckles how to resist eating candy after the two had become mermen.
Low Tidings Mermen
The mermen are like elves to Poseidon but instead of making toys, they punish the bad people of Stormalong Harbor by "sacking" them, meaning that they catch the bad people, stuff them in sacks, and beat them. The only escape from the mermen is to hide in a barrel until Low Tidings is over.
As they have fish tails and no legs the mermen are unable to move about very well on land (though they seem able to stand on their tails and wriggle along). However, when K'nuckles try to run from the mermen on solid ground, the merman form a giant man by wrapping their tails around each other in a fashion reminiscent of the Voltron force.
It appears the mermen in Low Tidings are the same as the ones that guarded The mermaid Queen.