The Longshanks, also known as Homo Sapien Elongatus, are a recognized strain of Abhuman within the Imperium and reside on worlds with low levels of gravity. In appearance, the Longshanks are bald with very pronounced eyes and have unnaturally long legs, when compared with baseline humans.
They are basically the reversed Squats, tall, lithe and physically weak. They used to be quite unknown in terms of their fluff, being put at the bottom of the barrel with the likes of the Troths, Neandors and Pelagers. However, they got some fluff with the War of The Beast novel, in which they were hilariously killed off by the Black Templars of all Space Marines.
Known as Orin's Well, this world had a population of Longshanks that was overrun by the Ork forces of the Warlord known as The Beast. Afterwards, thousands of Longshanks were taken by the Orks as slaves to work on Attack Moons. At the battle over Terra during the War of the Beast, Longshank slaves were at once executed for their impurity by the Black Templars who "freed" them. Technically making those Black Templars renegades and traitors as the Emperor ordered abhumans to be accepted and those Marines violated that order. It is hilarious to imagine they may have been purged by their own brothers after returning from their mission.
Despite this, Longshanks are known to serve in the Imperial Guard. A Guard regiment known as the Droonian Longshanks fought in Zodiox Rift in M32. Their tall height and weak physicality would have driven the Administratum interns in charge of logistics to the mother of all bureaucratic nerd rage.