Slithermorph
Slithermorphs are a species of Ooze appearing in the Forgotten Realms setting. Slithermorphs differentiate themselves from the multitude of other oozes and slimes by possessing two forms. One is their "non-combat form", appearing like a Black Pudding, and second is their "combat form", a portion of the Ooze rises up from the ground and begins to solidify into a snake-like form, with four arms resembling a Yuan-ti (whether this is a form of enhanced mimicry or a intentional feature by a unknown creator is not clear) but the form is powerful and dextrous enough to use weapons and tools. With an intelligence between 5-7, Slithermorphs engage in highly developed lives, they engage in land clearing in their territories along with the domestication of animals and agriculture of plants. With a neutral evil alignment, it can be assumed that Slithermorphs can be reasoned and even traded with should opportunity present itself.