Psirens are a breed of genetically engineered life forms, presumably originating as weapons. Their name is most likely derived from the mythological creature known as the "Siren" which was said to lure ships to their destruction.
The true form of a psiren is that of a gigantic insectoid creature. However, they have telepathic capabilities which allow them to create illusions in order to tempt their pray.
When a psiren encounters a potential victim, it uses its telepathic abilities to read their mind and determine the best way to attact them. It then alters their perceptions so that they believe they are seeing the necessary illusion, lures them into a false state of security, and then sucks out their brains with a straw.