Basilisks

Basilisks are a kind of carnivorous reptile, native to the jungles in the southern regions of far eastern Essos. Basilisks are venomous, and Basilisk venom is a notable poison used in assassinations.

In the books
In the A Song of Ice and Fire novels, basilisks are large, roughly dog-sized venomous reptiles that live in the jungles of Yi Ti, far to the southeast of the Dothraki Sea, across the Bone Mountains. They may also infest the tropical Basilisk Isles off the northern coast of Sothoryos, and also on tropical islands of the Jade Sea (which Yi Ti is on the northwest coast of), the same islands where poisonous Manticore insects are also found.

While wild basilisks originated in Yi Ti (and possibly other regions around the Jade Sea), they became a very popular animal for use in Fighting pits throughout Essos, made to fight other animals as entertainment. Thus they have been imported into many animal menageries across Essos. When Daenerys Targaryen passes through Qarth she sees a basilisk tearing apart a dog in a fighting pit; they are probably also found in the popular fighting pits in Slaver's Bay.

Exactly what a basilisk looks like is unclear, or if it is even meant to be analogous to a specific kind of real-life reptile. Because basilisks have been seen set against dogs in fighting pits they would seem to be roughly dog-sized, and therefore smaller than the massive Lizard-lions founds in the swamps of the Neck in Westeros. Due to their relative size and possession of poison, they may be somewhat like Komodo dragons.