Xen Tree

A Psuedotentacle (Literally "Small Tentacle") is a smaller relative of the Half-Life Boss creature, the Tentacle, and a native plant of the border world, Xen. Psuedotentacles are around thirteen to sixteen feet tall, brownish tree-like creature with no leaves of branches, a fully mobile "trunk", and a large, hook-like appendage at the top, used for attacking nearby prey.

They are scattered around the Xen levels in Half-Life and it's expansions as scenery, but can prove a danger to unwary players and Xen wildlife by violently stabbing at any nearby source of movement.

Physiology and Lifestyle
Pseudotentacles appear to be carnivorous, like their larger relatives. They attack nearby enemies so as to kill them and allow their bodies to decompose onto the surrounding ground, the nutrients from this process effectively feeding the Pseudotentacle. They are often found near Xen's Phosphorescent Stalk plants, and though it is possible this is a coincidence, it may also be a kind of mutualistic symbiosis; the light from the stalk attracts prey, then the Pseudotentacle kills it, feeding both plants.

It is possible that Pseudotentacles are simply the stalks of a much larger underground creature, just like Tentacles are.