Tentacle is a mermaid in the Northern Sea who is encountered during the quest, Deadliest Catch. She resides on a small island with two other mermaids named Urchin and Anemone. She and the others use the Thalassus as protection against humans and enjoy feeding humans to it.
When on the island, currently the only place to view Tentacle, it is impossible to use the Orb of oculus, and Tentacle tells you to pay attention.
There was a great deal of controversy with the release of Deadliest Catch, owing to her, along with her sisters, being the first NPCs to be partially nude in RuneScape, due to her uncovered breasts. She has since been graphically altered.
As an answer to all of the commotion about Tentacle's breasts being uncovered and visible, Mod Mark stated that this is not inappropriate, as it still fits within their "self-imposed rules".