The Xesto were creatures that consumed time and any form of temporal energy, including possible futures. Like Chronovores and Reapers, they were found in Gallifreyan legend. This meant that they could devour entire populations of any species in the universe, feeding on the amount of temporal energy they had. The Xesto were twice as large as a human, and either resembled a large weevil or worm.
Millions of Xesto inhabited the Tick-Tock World, regularly engaging in a "purge" that would strip the planet of any surface structures. When the unusual properties of the planet would cause a space-time vessel to crash on it, the Xesto were drawn to the crew's temporal energy, devouring them. Managing to survive the horde, Horl and Katta fed on Xesto meat, though reluctantly as the flesh held the memories of those the beasts had devoured.
When the First Doctor and his companions crashed on the planet, he, Ian Chesterton and Barbara Wright were all consumed by the creatures. When Susan Foreman found herself as the only survivor, she sought refuge in a Zero Cabinet but was barred by a future version of herself who insisted she throw herself to the Xesto and die a quick death. The Doctor intervened and suggested that his granddaughters instead exploit the Blinovitch Limitation Effect. Catapulted back to the moment of the crash, Susan was able to direct the Doctor in how to avert it. (AUDIO: Tick-Tock World)