Vanarx, also known as "Yeerk-bane", are long purple tube-like creatures that have a round mouth full of teeth and bodies like a raptor (dinosaur, not avian). They are native to the Yeerk Homeworld, and appear to be the Yeerks' only natural predator. A vanarx will latch itself onto the head of a Controller, then suck out and eat the Yeerk inside. Visser Three morphs a Varnarx in Book #2: The Visitor as a means to eliminate useless subordinates without killing or hurting the host-body.
Vanarax resembles a long, thick, purple tube that ends in a mouth filled with thousands of tiny suckers.