The Aldebaran serpent is a reptilian lifeform native to the Aldebaran system. It has three semi-transparent cobra-like heads extending from a brilliantly glowing sphere hovering above the ground, surrounded by lights. Q briefly assumed the form of an Aldebaran serpent when he was introducing himself on his second visit to the USS Enterprise-D in 2364. (TNG: "Hide and Q")
In the Q Continuum miniseries, Q turns Picard into an Aldebaran serpent during their initial trip into the past, saying that it was one of his favorite forms before humanoids began to evolve; he claims he did it to create a sense of antiquity. Picard's mind can't handle three viewpoints at once, so Q reluctantly changes him back.