L'Huraxi
The L'Huraxi is one of many, many, MANY xenos species that're briefly mentioned in 40k's fluff and promptly ignored afterwards. L'Huraxites are apparently humanoid giants with purple skin, and are at least capable of developing a certain degree of engineering, as their use of massive attack vehicles proves. They fought against the Catachan Jungle Fighters led by Straken at the Battle of Moden's Ridge, though the result of that fight are inconclusive (as in, the text is in the 3rd Ed. Catachan Codex, which is coincidentally the only mention of the L'Huraxi, but I can't find the actual text anywhere, if someone can find it please add it here).
"That ain't proper armour like you'd see on an Imperial tank - no it's flimsy, fall-down-in a-strong-breeze, I-hope-they-don't-spit-at-me armour." - Colonel 'Iron Hand' Straken.
Basically, imagine an army of Thanos riding tanks and fighting Catachan. It would be awesome if we actually had more info than that.