From A Wiki of Ice and Fire
From a distance squishers appear human, but according to legend their heads are larger than those of men and they have scales instead of hair. They have webbing between their fingers and toes, while their mouths have rows of green, needle-like teeth. A squisher's belly is white like that of a fish. Their name comes from the squish-squish sound made when they move on their webbed feet.
Some or all members of House Borrell have webbed hands and toes similar to the feet of ducks, which they call the mark. It is unknown if this is a genetic disorder or if they have a connection with squishers.
A Feast for Crows
|“||Dick: They'd eat you, boy. They'd eat you raw.
Podrick: If they try, I'll kill them.