Vomitus is an unused Quake enemy. It was a large blob that was intended to throw up smaller versions of itself (Spawn) to attack the player. The only information about it comes from the model in Qtest and from a interview focusing on John Romero for Quake's 10th birthday. 
The Vomitus seen in Qtest appears to be a relatively large blob, when killed its mass will lower into a pool of liquid. The Vomitus relies entirely on melee damage, moving towards the player and attacking with a slow-moving bite. Note that this bite causes no damage, making the Vomitus entirely harmless. They appear to have a similar amount of health to the Death Knight.