A three-headed serpent associated with Aldebaran, whose shape Q adopts inside a glowing sphere when he intercepts the Enterprise-D at the start of the episode Hide and Q.
The Aldebaran serpent is a creature with three cobra-like heads rising from a single body. It is known from one appearance, and that one an impersonation.
In Hide and Q, the Enterprise-D is racing to help the survivors of a mining disaster on Quadra Sigma III when a grid blocks its path and a shining globe appears on the bridge containing a three-headed snake. Picard recognises the visitor at once as Q, who has chosen the shape of an Aldebaran serpent for his entrance.
Q quickly discards the form in favour of a Starfleet admiral's uniform and proceeds to test Riker with the power of the Q. Whether real Aldebaran serpents are dangerous is never established.