Treant
Treants (or sometimes just "ents") were fleshy, amorphous blobs of yellow and pink flesh that oozed through forests. Their slow movement made them easy prey--at least, until they carved themselves wooden armour.
Treants would often pick up stray pieces of bark and broken logs as they rolled across the ground, using the debris as a shell. They'd also often eat their way into thick-trunked trees and worked until the plant was separated from the ground. From there, they would break the tree down into more maneuverable parts for a more sturdy shell.
These bizarre things were usually afraid of putting up a fight due to their soft, easily damaged bodies. However, once encased in a suitable wooden shell, a treants would viciously defend its territory to protect weaker familial treants from potential predators.