Mire Beast

Mire Beasts were octopus-like creatures native to Aridius. They originally lived in the oceans, but these dried up, forcing them into the Aridians' cities in search of food. The Aridians kept out the Mire Beasts by walling off and blowing up the portions of the city infested with them. (TV: The Chase)

Circa 59,000 BC, the Gubbage Cone empire kept Mire Beast fighting pits. (PROSE: The Crystal Bucephalus)

When the First Doctor visited Aridius, one Mire Beast attacked Ian Chesterton and Vicki. (TV: The Chase)

Behind the scenes
According to non-narrative information in DWBIT 41, when the Daleks later attacked Aridius, they disturbed the Mire Beast, including the Mire Colossus. The Mire Beasts were destroyed, allowing the Daleks to destroy the Aridians.