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 but lightly-armoured attack vehicles proves. They fought against the Catachan Jungle Fighters led by Straken at the Battle of Moden's Ridge as part of the Yaquit 27 campaign, though the result of that fight are inconclusive (as in, the text is in the 3rd Ed. Catachan Codex as Straken's background lore, which is coincidentally the only mention of the L'Huraxi, so see Straken's page for the full quoted passage).

"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.