A degenerate race of frog-like horrors, with ties to even greater, unspeakable forces.
In the Prohibition Era, the Batrachi had several run-ins with the Blasphemy Boys. While the creatures' home was somewhere in Ipswich, they had been known to venture into the larger world on occasion.
One such instance was when they ambushed the Blasphemy Boys in the sewers under Baltimore. It was a disastrous outing for the human group, who lost all but three members. ("Thumbtacks & Yarn")
Their appendages, like those of the tree frogs they resemble, allow them to adhere to surfaces.
The Batrachi clearly owe much to the works of H.P. Lovecraft. In addition to the tonal similarities, Lovecraft's work frequently dealt with dark, unfathomable forces from beyond, which often originated in New England, and it frequently used the word "batrachian." (It means "frog-like.") Furthermore, Ipswich has been speculated to be the real-life inspiration for Lovecraft's Dunwich.