Kroll

Kroll was a gigantic creature on Delta III, one of the moons of Delta Magna.

Origins
Kroll was descended from one of the giant squid brought to the moon by the humans. At one point, it ate one of the priests of the Swampies and their holy relic, which was actually part of the Key to Time. This segment mutated Kroll, making it grow to a mile across and a hundred forty feet high, with over thirty tentacles on each side of its body. Though it was mainly a herbivore, it also ate any animals it could sense by vibrations. Kroll spent most of its time under the mud of the moon, passively feeding on plants and animals. It surfaced at least three times, becoming the god of the Swampies. The Swampies called these scattered appearances "manifestations".

Fourth manifestation
The fourth time it surfaced, the moon was inhabited by humans searching for methane. Kroll produced large amounts of methane during his periods of dormancy. This was harvested by the humans. Kroll was woken by the drilling and the firing of the orbit shot.

At this point, the Swampies were trying to take back the moon from the humans and took Kroll's appearances for a sign, even though Kroll ate humans and Swampies indiscriminately. The Swampies grew bolder and attacked the methane plant, killing Thawn. Unfortunately, this commotion attracted Kroll and it attacked the refinery. Arriving with Romana during their search for the Key to Time, the Fourth Doctor converted Kroll back into a segment of the Key, leaving only a few giant squid behind; according to him, it was the segment that gave Kroll its powerful state, meaning that there would "never be another Kroll" so long as it remained off-world. (TV: The Power of Kroll)

Behind the scenes

 * According to the publisher's summary for PROSE: Time Traveller's Diary, the Second Manifestation took place on 12 December 2528, but this information does not appear in the book itself.