Decapus
by Robinfirst appearance in a Vampirella comic 1971 as a semi-sentient tentacled tree monster of Horror and
The decapus was originally created in 1981 by Jean Wells for the infamous B3 adventure module Palace of the Silver Princess, which took place in the Mystara setting
Clearly the similarities reveal the source
That said
The Decapi can speak their own tongue (a non-verbal way of speak using their tentacles)
They thus would more easily understand the non-verbal speak of the Deaf/Mute people.
They can however shapechange into a humanoid form by bringing fort a powerful illusion(which even clouds Second Sight of Fairies and Dragons as well as any Treusight or similar spell), where they can speak verbally (if at least learned a language, which takes 3 months minimum and then an intelligence check, or add another month until the next check in that humanoid form)
If they have not learned any language their vocals are similar between a shriek and a growl.
The tentacles btw can stretch to a reach of triple the decapus' normal reach(10 feet+1 feet per CO bonus, and the body is 1/3rd of this creature's relaxed size. An 18 Constitution(+3) would thus have a body of 1/3rd of 13 feet =4 feet and could stretch to grab/hit up to 39' away, and this does not need to be a straight line. Like Octopi, they can sense an direct their tentacles even if blinded, around any obstructions, and like octopi they taste even with these if a prey is living, dead, edivble, tasty.
Decapi are rumored (Fanon) to originate from the Nightmare Realm (which befits its original source of Vampirella's haunted forest/ruins where it was depicted), and called to Mystara by Nithians, Taymorans, Alphatians, and recently Glantrians and evil Hags worldwide.
I myself use these as yet another Nightmare realm creature, related to the Neh-Thallgu, yet not befriended with these. In their own realm I let them feed on Diaboli, and as Humans, Demihumas and Humanoids look somewhat similar, they prefer these as food here
They are renamed as Canopy lurker by Maze of Death Pathfinder Battles with a nice miniature
I see the Marine variety as equal as the normal, just they adapted to the environment when appearing first on Mystara
They are oviparous, requiring chaos as a nesting/hatching area