Didoan

The Dido people were a peaceful humanoid race native to Dido.

History
When a spaceship crashed on the planet in 2493, the Dido people met with the surviving passengers, but were killed along with them by Bennett to cover up a murder he had earlier committed. At least two of the Dido people survived, and when they confronted him, he backed away in fear and fell from a rock ledge to his death. (DW: "Desperate Measures")

Behind the scenes
The Rescue doesn't give us a clear, one-word demonym for "a being from the planet Dido". The name, Dido people, is what the First Doctor directly called them in "Desperate Measures". It's possible that a grammatically acceptable demonym would be Didonian — but equally logical are Didoan and Didoite.