The Krattorians are a purple / blue skinned species which operate as slavers
A Krattorian ship carrying human slaves made the mistake of landing on Skaro in search of minerals from the Skarosian sand.
Employing their usual blend of trickery and treachery, the Daleks were able to exterminate the Krattorians.
The Krattorians featured in an early chapter of “The Daleks” (AKA “The Dalek Chronicles) first published in the comic TV21 during the 1960s.
This story / episode is “Power Play” and originally ran in TV21 issues 4-10 (7 pages total) from 13 February – 27 March 1965 and was later reprinted in :
1) The 1973 Doctor Who summer special in black and white
2) Doctor Who Weekly 33-34 in black and white
3) Doctor Who Magazine 183-188 in colour
4) Doctor Who special “The Dalek Chronicles” in colour
It is possible, considering their similar appearance and that both species landed on Skaro (presumably from nearby worlds) that the Krattorians are related to the Monstrons.
Although the Krattorians did not reappear, there was a reference to “Krattorian dhushi” (a blue food resembling custard) in “The Hungry Bomb” a story from ” The Doctor Who Yearbook 1995″, written by Gareth Roberts and featuring the 3rd Doctor and Sarah-Jane Smith.