A range that is much confused both by releases with the same number, such as the WF6 "Aardvark" and WF6 Chaotic General Manager attacking with Mace, and by differences between UK and US releases. As far as I can tell, WF1-10 were originally released by Citadel on the UK, with the WF6-9 Aarvarks withdrawn for copyright reasons and replaced in September 1982 by the WF6 GM and WF7 Golem.
I don't recall seeing any Citadel UK catalogues that list WF11 up, but they are in the 1983 Citadel US and 1984 Ral Partha Imports ones. Ral Partha / Citadel Miniatures US added extra codes by repurposing some existing Citadel miniatures (WF11, WF13, and WF14). They resculpted the range once the split with Citadel was final.
WF 1-5 first appeared in WD22 February / March 1981, WF10 in the June 1982 flyer
Aardvarks in the UK and Earthpigs in the US, they had been replaced in the UK by October 1982 when the GM & Golem were announced
Listed on the September 1982 flyer
Most of these were in different ranges in the UK