D&D Monsters: Treants

Although the idea of magical and possibly animate trees does exist in mythology and folklore, the idea of a race of such beings, in the form of ents, seems to be original to Tolkein. He took the name from a variant form of  "ettin", the original English word for what we now call a "giant", and not from any pre-existing tree-person. (Other giants exist in Tolkien's world, but they are very much bit players unlike, say, the giants of Narnia). D&D 0E included ents in its original rulebook, but legal problems led to them being renamed as "treants" from 1E onwards. For similar copyright reasons, it's this newer name that has stuck in fantasy games in general, even though, really, there isn't much difference between the D&D version and their entish inspiration.

D&D Monsters: Xorns

Xorns are an original creation of D&D, with no particularly obvious antecedents. The name was almost certainly chosen so that the 1E Monster Manual had at least one monster for every letter of the alphabet... the same is probably true of the quasit. Which, other than that it's a monster largely built around defence rather than offence, is about all one can say about the general concept.

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1E

The original xorn has a barrel-shaped body covered with scales that basically take the form of roof slates, and are likely made of a similar, if less fragile, material. It is notable for its (mostly) triradial symmetry, something that makes it look particularly alien. It has three legs, which seem to lack knees and are similarly covered in the armoured stone scales - if anything, it's a wonder it doesn't move even more slowly than it does. The arms look to be less heavily armoured, and are clearly more flexible; they're positioned above the spaces between the legs rather than directly above the hips.