Grumpuses are the dominant species in the world of Bugsnax, acting very much like humans in our world. They are bipedal, somewhat cylindrical creatures covered in fur, the color of which can vary greatly between individuals. Each Grumpus also has a unique nose shape and color.
Grumpuses have large mouths with large teeth, perfect for eating Bugsnax. When a Grumpus eats a Bugsnax, a strange transformation occurs, in which a part of their body changes into the food that Bugsnak is made of. Their bodies, noses, limbs, hands, and feet can all transform in this manner, and in some cases, accessories such as "horns" and "hats" may even appear. This phenomenon is assumed to be a property of the Bugsnax themselves, not exclusive to Grumpuses.
The player takes the role of a Grumpus journalist working for GNN. Their fur color can vary depending on the color of the save file they selected. They are allergic to Bugsnax, and therefore unable to eat them and transform their body.
Elizabert's Expedition Team
Mentioned Grumpuses that do not appear in the game. They include:
- Bronica Lottablog- Triffany's grandmother, an explorer who greatly inspired Triffany, and who ventured to Snaktooth Island herself at some point. Photos of her can be seen in a scrapbook on Triffany's desk.
- Grumpbeard- A pirate who mysteriously disappeared from history. Triffany deduces that he is one of many who shipwrecked in Boiling Bay long ago.
- Beffica's ex-friend'- A blue Grumpus who was a friend of Beffica in the past. They can be seen in a photo in Beffica's hut.
- Stone Grumps- An ancient group of Grumpuses who lived in the Flavor Falls/Garden Grove area. They created stone statues depicting both Bugsnax and Grumpuses, but no remains of their bodies are known.
- Desert Grumps- An ancient group of Grumpuses who lived in the Scorched Gorge/Sizzlin' Sands area. They created sandstone houses and ziggurats, as well as pottery and some sculptures. Their skeletal remains are found throughout the area.
- Filbo was the first Grumpus designed, and his design was used as a base for the others. The main basis for his design was "a little flip top trash can creature".
- The first names of Grumpuses are made by substituting letters from a common first name until it is barely recognizable. The surnames are made by combining a fun noun or adjective with a fun noun or verb.
- Each Grumpus has at least one accessory, to remain visually distinct even with a body made of snacks.
- A post by the Young Horses Twitter account revealed the earliest model of a "Grumpus", which has a similar body shape, though with small horns and a much smaller face. Its silly appearance quickly made it popular both on Twitter and in the Young Horses Discord server, where an emoji of its face, named "folbo", was added.