Often derided for resembling 'Hobbits in Space', Ratlings (Homo sapiens minimus) have swapped magic rings for sniper rifles, needlers and Long-Las, all the better to serve in His Most Glorious Imperial Majesty's Imperial Guard. The friendly wizard Gandalf has been replaced by a Primaris Psyker with a thirst for pint-sized auxilia (which admittedly is a bit of a loss); thankfully, Ratlings are excellent at both self-preservation and cookery, presumably fending off their enemies with a massed barrage of just-baked bread rolls. This is only to be expected, since 40K began as Warhammer Fantasy IN SPACE! and thusly Ratlings are basically Warhammer Halflings with lasrifles. Also, keep them away from the armory. They tend to eat the laspacks.
Has nothing to do with the Skaven.
Ratlings are a breed of abhuman, putting them in the same dropship as Ogryns and their not-to-be-named brethren, and they are about as popular with the fanbase as they would be with the citizens of the Imperium. They measure three feet in height, 50% of which is stomach - they can eat twice the ration allowance of a regular Guardsman, presumably in a first, second, and third breakfast. Unlike their fantasy counterparts they disdain adventure, displaying an eagerness to get into cover that in any other squad would be labelled as 'shameful' but for these waist-high warriors should be referred to as 'characterful'. While sociable creatures, make sure you don't leave anything precious lying around, as Ratlings tend to loot anything that isn't nailed down (and a few things that are (and the nails (...and whatever they were nailed to))). This is offset by the fact that they can make a decent meal out of anything, including Departmento Mutintorium nutricrud. Another interesting thing to note about Ratlings is that they breed like there is no tomorrow. Possibly because there might not be.
In the game
Ratlings make a good addition to blob based lists. With BS 4 snipers and free camo cloaks, though 7th ed Sniper weapons lost the true rending ability against vehicles they still function at AP2 on 6s to-wound, so Ratlings can piss off smaller high value units or monstrous creatures and free up that extra shot for your Leman Russ to Pie-plate some Grey faggots. Throw something to take the shit or put them in a building and for just 100 points, watch 10 furry midgets glance shit to death. Oh and I almost forgot; Ratlings are one of the worst units for assaulting with (how bad are we talking? Fire warriors will take their lunch money every time), so keep anything that could charge them very far away or see Frodo be stabbed to death (that could be quite cool).
Nowadays in 8th Edition, Ratlings are... still pretty solid. They make tissue paper look robust at Toughness 2, but they're BS 3+ now, sniper rifles got a buff, and their cover stacking keeps them alive. They're a fairly strong sniper unit that kept move-shoot-move (which most units that had it already lost) and do well enough at character killing. For a bunch of hobbits with rifles, not bad. They also finally got (two!) named characters through Blackstone Fortress, though they're not exactly useful. While their rifles have no AP, when used against Genestealer Cults or other Guard units 10 split firing snipers can be infuriating.