Monster Donut is a chain of doughnut shops across the United States.
The shops are connected with the life force of the Hydra and many other monsters. This was stated by Annabeth Chase as being the reason some stores pop up so fast. This trick was discovered by some children of Hermes in the late 1950s. Another Monster Donut shop pops up somewhere every time the Hydra grows two heads (after losing one). It also seems to be magic as the shops grow out of thin air, and Tyson thought that their doughnuts were exceptionally good.
After Percy Jackson, Annabeth, and Tyson escape the Princess Andromeda, they hide in an old hideout created in the days when Annabeth, Thalia Grace, and Luke Castellan were on the run. Wanting some privacy with Annabeth, Percy sarcastically tells Tyson to go out and grab some doughnuts. They are surprised when he comes back with a box of doughnuts, claiming that he found them fifty feet from their hideout, so they go explore the area and see that there is a Monster Donut shop nearby. Apparently, only monsters are able to purchase the doughnuts (which is the reason why Tyson was able to do so) while demigods are killed and it appears partly because Tyson wished it. The Hydra eventually connects to Monster Donut then appears and attacks the trio. They are saved by Clarisse La Rue, who uses a cannon from the Confederate Ironclad battleship, CSS Birmingham, to slay the Hydra.
Monster Donut sponsors a section of the Georgia Aquarium owned by Keto and Phorcys. While Percy Jackson, Gleeson Hedge, and Frank Zhang are being given a tour by Kate, Percy saw a red sign which he had to read twice because of his dyslexia which said this section was sponsored by them, reminding Percy of the first time he had to fight the Hydra. Later, dougnnut-shaped circles that smelled like doughnuts made out of smoke appeared when they escaped from Phorcys and Keto.