The frog monsters are a race of sapient humanoid frogs that are created from humans affected by the power of Sadu-Hem.
As the Ogdru Jahad advance their plans on Earth, it becomes evident that they intend to introduce a new "final" race of man, in the form of the frog-monsters. The monsters are extremely powerful, both physically and psychically. A single creature was able to overpower Hellboy, though this specific frog monster appeared to be stronger and more powerful than the general population. The frogmen are able to shift between their human and frog forms, allowing them to pass unnoticed in populated areas. Moreover, the frogmen can exert a degree of telepathic influence over their victims, confusing them with illusions and mind-tricks. Finally, the touch of one's tongue is extremely painful and leaves pock marks on a victim's skin.
The frogmen are linked to the Ogdru Jahad, and are being employed to free the Ogdru Hem who were imprisoned by the Watchers at the beginning of time. Once the Ogdru Hem are free, the Ogdru Jahad will follow. Currently, numerous frogmen have escaped the clutches of the B.P.R.D. and are moving across the North American countryside. These nests seem constructed around the purpose of praying to the Ogdru Hem, and in certain locations relics and items have been found which spawn vicious and lethal Ogdru-Hem-like creatures. Moreover, the frogs have transcended their human limitations and have developed the ability to convert their bodies to biological factories, producing thousands of tadpoles which become frog monsters.
For a time the Zinco Corporation had several frog monsters in their R&D department. They had captured these creatures when they were tadpoles and were researching the monsters in secret. One interesting discovery Zinco was able to learn was about the frog monster's language. Their nests and hideouts usually contained some sort of primitive altar to the Ogdru Hem and a mysterious language was usually carved throughout the area. Zinco was able to discover that the frog's language is somehow instinctual. That is, the frog monsters never needed to be taught their own language, knowing it was in their DNA.
There appear to be variations amongst the human turned into frog creatures depending on which Ogdru Hem was the cause of their transformation. Not all Ogdru Hem are capable of creating frog monsters.
The main frog monsters seen in Hellboy and B.P.R.D. have been the spawn of Sadu-Hem. However, the Conqueror Worm entity was also able to turn men into frog monsters during the events at Hunte Castle. These frogs looked different and were changed in order to be devoured by the worm as it raged across the land. All of these frog monsters were devoured by the worm and the Ogdru Hem was killed by Roger the Homunculus before it could create anymore.
After the destruction of Katha-Hem, the frog monsters retreated underground taking Pope, the now reluctant Black Flame with them. They soon encountered and forged an alliance with the Servants of the Left Hand and their leader, the King of Fear. The Servants of the Left Hand are a race of underground humanoids bred to be slaves by the People of the Left Hand; Hyperboreans who worshipped the Black Goddess and retreated underground after the fall of Hyperborea. The Servants eventually revolted and massacred all of the People of the Left Hand.
After the defeat and destruction of the majority of the frog forces and their allies, The Servants of the Left Hand, the few remaining frog monsters fled. These worked with the Black Flame, the King of Fear, and his army of Servants of the Left Hand. Their goal was to try and change the frog monsters into massive destructive creatures by bonding them with the machines of the old Hyperborean empires. However, these last remaining frog monsters were killed alongside the Black Flame and the underground mole-men by the massive wave of destruction unleashed by B.P.R.D. Agent Liz Sherman. Since the Black Flame claims that the frog monsters present were the only ones remaining it appears that the race has gone extinct.
The sole frog monster kept captive by the Zinco Corporation was killed by the revived Black Flame.