Sandy was a sand beast befriended and tamed by Vicki when she arrived on Dido. He lived in a cave close to Vicki's ship. Vicki liked to spend time with Sandy, stroking his scales. He growled when she did this. (PROSE: The Empire of Glass) Sandy was killed by Barbara Wright with a flare gun when she mistakenly thought he was about to harm Vicki, which upset her. (TV: The Rescue)
When the Monk caused the Fifth Doctor to swap places with the First, the Fifth Doctor convinced Vicki and Steven of his identity by mentioning the circumstances of Sandy's death, something that Vicki could be sure nobody outside of herself, Ian, Barbara and the Doctor would know (AUDIO: The Secret History).