The Frog Monsters are recurring antagonists in the Hellboy-universe, who are inspired the Deep Ones.
The Frog Monsters were all once humans, who had been turned into creatures by the Ogdru Hem.
The creatures were first met by Hellboy, Abe Sapien and Liz Sherman in Cavendish Hall in the 1990's, where Sadu-Hem had turned four humans into these creatures. The were all killed by the B.P.R.D.-agents.
The Conqueror Worm also turned people into Frog Monsters, which were then consumed by the Worm.
Katha-Hem also turned people into Frog Monsters during the apocalypse.
- Besides the Deep Ones, these creatures are also likley based on the second plague of Egypt (Ex. 7:25-8:15) and "three impure spirits that looked like frogs" from the End times (Revelation 16:13).