Walter the Worm

Walter the Worm was a sentient worm that lived on Earth.

At some point in the 20th century in London, he was in a hole and expressed discomfort when Susan Foreman played pop music on her radio loudly. When the pop music repelled the approaching Cybermen back into their spaceship, he expressed happiness, as did Susan and the First Doctor. (PROSE: Dr. First)

Sometime in the 1970s, Walter was in a hole on the side of the road when the Third Doctor rode in Bessie. Mike Yates' Land Rover had broken down so the Doctor stopped next to Walter to let Mike in. The Doctor then set off again with urgency as he intended to enjoy ice cream in his laboratory on the hot summer day before it melted. (PROSE: Dr. Third)

At one point, he was chased across a hillside by Daleks. Somehow, he escaped extermination. (WC: The Daleks Chase Walter the Worm)

At some point in the 21st century, he was sitting in a hole when the Fourth Doctor and Sarah Jane ran past, running from a group of Daleks. Shortly after this, Walter too, left his hole to flee from them. Once again, he narrowly escaped death. (PROSE: Dr. Fourth)

Later, he was sitting at the top of a ravine in a wood. The Seventh Doctor had climbed the ravine from the undergrowth below and was peering over the rock face at Walter and the figure behind him. The figure was in fact. By the time that the Master had announced his presence and started advancing towards the Doctor and Ace, Walter was gone. (PROSE: Dr. Seventh)

On the day before Yasmin Khan's birthday, Walter was sitting happily in a hole next to Yaz, who was excited for the occasion. She wondered what the Thirteenth Doctor, Ryan Sinclair, and Graham O'Brien had planned for it. (PROSE: Dr. Thirteenth)

Walter had a counterpart on the Blue Planet. The Blue Planet was a planet that was very similar to Earth apart from the fact that everything was blue, including Walter. This Walter seemed to be indifferent about sitting in holes and was also slightly longer than the other Walter. When the Sixth Doctor and Peri Brown were exploring the planet, he was happily slithering along on the ground next to them for some of their walk. (PROSE: Dr. Sixth)

Behind the scenes
Walter the Worm is a character from the original run of Mr. Men books who regularly made small cameos, often simply sitting in a hole.