A Tentacle is part of a five part swamp creature encountered during the Temple Trekking activity on the hard path. It has a max hit of 12. It is recommended to poison a tentacle, as that will poison the whole monster. If all four tentacles are not killed before the head, the head will regenerate.
The Head is level 140 and its four tentacles are level 99 each.
While rising, each of the four tentacles appear as level 136, and once they're finished rising, they appear as level 99. It is unknown whether this is a glitch.