I was thinking of using a giant octopus for an encounter. It's attack reads:
Tentacles: Melee Weapon Attack: +5 to hit, reach 15 ft., one target. Hit: 10 (2d6 + 3) bludgeoning damage. If the target is a creature, it is Grappled (escape DC 16). Until this grapple ends, the target is Restrained...