A Grimy Creature is a grimy herb that can be cleaned (doing so will turn it into a Clean creature) in order to turn it into a Creature combat potion by using it with a water-filled vial. Creature combat potion brewing is exclusive to Legends but drinking the potion is available to other players as well. Brewing the potion will always result in a 4-dose potion. In order to clean a Grimy creature, a player must have a Herblore level of 90 or higher. The 'clean' option is available to all players with a Herblore level of 90 or higher and will grant you 5000 (Sir), 2000 (Lord) and 300 (Legend) Herblore experience.
Grimy Creatures can be obtained by defeating high-level monsters, such as a Giant Roc or a Hellhound. They currently (Mid-September 2014) sell for 200.000 - 300.000 gold pieces. Some Legends may turn your clean/grimy creatures into Creature combat potions, however they might ask you to pay them to do so.