Scalies more predominantly hail from the Underhive of Necromunda and can be loosely described as "Ogryns turning into lizardmen". More specifically they are a mutant strain of Abhuman found in the underhive which have stabilised over time, although there is still a lot of variation, and are not as stable other abhumans found in the Imperium, like Ogryns and Ratlings. While they may look like Killer Croc, they differ in that Scalies don't often eat people.
Scalies are unique in that as mentioned, are massive reptillian mutants almost as broad as they are tall. Unlike conventional humans or mammals for that matter, Scalies seem to have adapted to have almost reptilian features. Their durability and strength are legendary among the people of the Underhive and have a fearsome reputation. Like some reptiles they have the abillity to regenerate lost limbs and their thick skin acts as a natural armor.
They are rarely seen but it is said that they are by far the most numerous denizens of the Underhive. They are undoubtedly intelligent (Although that really isn't a big feat compared to the raging retardation of the average Imperial Citizen) but seldom speak due to their twisted mutated necks and voice boxes having changed beyond the ability of normal human speech. So little is known about their origins or if there are hidden clans or tribes of Scalies in the most severely polluted badzones in the Underhive.
They are mainly found in the employ of Scavvies. Why they choose to work along side them is unknown. No Scavvie Boss or King worth his salt will be seen without one or two Scalies in their gang.
When fighting they often use close combat weapons and discuses or throwing axes. They are also used as heavy weapons men by Scavvies using portable bolt throwers called Spear guns or Scatter Cannons, a large form of blunderbuss.
They are featured and played in the old Necromunda Tabletop where they act as shock troops and the Underhive gang's equivalent to Ogryns.