One of my old Fiend Folio favorites, brought to the new edition. The flavor text is not mine, I just did the stat conversion. NOTES: Is it nuts that a CR 2 creature can have 55 hit points? That seems nuts to me. 5E, you have strange math.
Tentamort (CR 2; 450 XP)
Medium monstrosity, unaligned
Armor Class 12
Hit Points 55 (10d8+10)
Speed 10′, climb 10′
Str 15/+2, Dex 14/+2, Con 13/+2, Int 3/-4, Wis 14/+2, Cha 5/-3
Skills Stealth +4
Damage Resistances poison
Condition Immunities prone
Senses darkvision 60′, passive Perception 12
Retraction. The tentamort may compress itself and all of its tentacles into small crevasses in rocky, swampy, or otherwise suitable terrain. Doing this gives it AC 15 and advantage on Dexterity (Stealth) checks but renders it immoble.
Spider Climb. The tentamort can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.
Tentacle Sever. The tentamort’s tentacles may be targeted in combat. Each one is AC 12, 15 hit points. Damage done to a tentacle counts against the creature’s total hit points. A severed tentacle is destroyed and cannot attack. It regenerates severed tentacles over the course of three days.
Multiattack. The tentamort makes two attacks, one with each tentacle, or two with its poison tentacle against a grappled target.
Grasping Tentacle. Melee Weapon Attack: +4 to hit, reach 15′, one creature. Hit: 11 (2d8+2) bludgeoning damage and the target is grappled (escape DC 12) if it is medium or smaller. While grappling the target, the tentamort has advantage on attack rolls against it and can’t use this attack on other targets. The tentamort may attempt to push or pull the target 5′ per turn as a bonus action if it defeats the target in a contested Strength check.
Poison Tentacle. Melee Weapon Attack: +4 to hit, reach 15′, one target. Hit: 11 (2d8+2) piercing damage and the target must make a DC 12 Constitution saving throw or be poisoned for ten minutes. While poisoned, the target takes 9 (2d8) damage at the beginning of each of their turns and cannot recover hit points. The target may make a new saving throw to overcome the poison at the end of each of their turns.
(Text from the Pathfinder PRD.)
Tentamorts are eerie ambush predators, preferring to let prey come to them rather than seeking food out, and relying on their excellent senses to warn them of approaching meals. A tentamort possesses several tentacles, most of which are used for locomotion but two of which have evolved for singular purposes in securing food. One of these longer tentacles is covered with tiny, sticky nodules and is capable of constricting prey, while the other ends in a long, thin stinger. The tentamort’s method of attack is to grab its prey with its constricting tentacle and sting the grappled target with the other. Tentamort poison is particularly horrific, as it swiftly liquefies the creature’s internal organs into a rancid slurry the monster can then drink with the same stinger, siphoning out the fluid with foul sucking sounds. Larger creatures often require multiple stings (and multiple failed saving throws against the venom) before they can be fully absorbed by a tentamort. Tentamorts are almost mindless, possessing just enough intellect to make crude animal judgments about peril and food. Once a tentamort has grabbed prey, it tends to focus entirely on that creature, ignoring attacks upon it from other sources as long as its current victim remains a source of nutrition. After a tentamort finishes consuming a creature, all that typically remains are the bones and skin.
A well-fed tentamort uses the hollow corpse of its meal as a sort of incubator for its eggs, injecting the body with a caviar-like mass of black eggs that mature in the rotting carcass for several weeks until a dozen or so hand-sized tentamorts hatch and crawl out of their host’s orifices. Depending upon the availability of other prey, anywhere from one to six of these may survive, feeding on rats and Tiny vermin, until they eventually grow to adulthood. Tentamort young look like dark blue starfish with a single red eye in the center—they do not possess their longer, specialized tentacles until they mature. A young tentamort often attaches itself to a larger predator, clinging to it much the same way a remora clings to a shark, dropping off to feed innocuously on its host’s kills while the creature sleeps.
Some tentamorts grow much larger than their human-sized kin. Known as greater tentamorts, these ogre-sized creatures have at least  Hit Dice and are Large sized. Their two specialized tentacles grow to 20 feet long, providing the creature with greater reach than a Large monster normally possesses. Greater tentamorts are never found in groups, for these creatures can only achieve such monstrous size through cannibalism, as if there were some key nutrient in another tentamort’s body that allows them to exceed their typical physical limitations. Some of these creatures have mutations giving them two tentacles and two stingers. Yet the most disturbing quality possessed by these monsters is their unexpected intellect—greater tentamorts are often as intelligent as humans, or more so. They cannot speak, but possess an eerie form of telepathy that works only upon creatures they are in physical contact with—a feature they often use to “chat” with their food as they eat.