From Destinypedia, the Destiny wiki
The Vex Wardens are a group of Vex units who are seen deployed to prisons or testing grounds to eliminate riots or test. There are two variants of the Wardens.
The Minotaur variant of the Vex Warden are two Ultra Minotaurs seen in the Release Adventure on Nessus. They serve as riot enforcers as a means of instilling order.
The Hydra variant of the Vex Warden are two Major Hydras seen in the Lost Crew Adventure on Nessus. They serve as defensive constructs and field scientists to test Guardians and protect secrets.