From A Wiki of Ice and Fire
The sigil of House Gargalen
depicts a snake in the beak of a cockatrice.
Cockatrices are possibly mythical creatures of the Known World. Cockatrices from real-world legends are two-legged creatures with the body of a dragon or serpent and a rooster's head.
Among the grotesques and gargoyles of Dragonstone are cockatrices. The sigil of House Gargalen of Salt Shore includes a cockatrice.