User blog comment:BannedLagiacrus/Discussion of the Week: Invasive Species/@comment-12331904-20141105163956/@comment-12331904-20141110164406

not necessarily! there are free-floating goose barnacles that will sometime congregate in swarms that are sometimes mistaken for sea monsters. Those could be supersized and powered up into actual sea monsters in MH.

also buoy barnacle that form their own foamy rafts that sometimes gather their own, separate barnacle populations on them, once again forming a soemwhat intersting swarm-type monster.

Even if it's a single gigantic barnacle the movement could come from various offensive appendages rather than its body. Whips and claws sliding in and out of its armor plates. It could also reasonably only be cemented to the ground for part of the fight, breaking away, flipping upside-down and revealing hidden legs for the second leg of the battle.