These lizards appear to be tattooed, and run down their prey on their powerful hind legs. They also have long curved claws on their hind legs, which they use to rip their prey apart.
A Game of Thrones
A Dance With Dragons
Hear me. The ships of Braavos sail as far as the winds blow, to lands strange and wonderful, and when they return, their captains fetch queer animals to the Sealord's menagerie. Such animals as you have never seen, striped horses, great spotted things with necks as long as stilts, hairy mouse-pigs as big as cows, stinging manticores, tigers that carry their cubs in a pouch, terrible walking lizards with scythes for claws. Syrio Forel has seen these things.
Behind the Scenes
It is likely that the "terrible" walking lizards are a kind of dromaeosaurid dinosaur, such as Velociraptor or Deinonychus. "Dinosaur" means "terrible lizard", "dromeosaurid" means "running lizard", and they have a large scythe-like claw on their hind legs. Furthermore, George R.R. Martin greatly enjoyed the novel Jurassic Park, which popularized velociraptors as vicious killers.