The Ulitka is a grayish green snot-like slug type creature. They tend to be just over a foot long, and very dense. They lack any sort of visible sensory organs, and are rather alien in nature. While frequently referred to as snails, they are slugs with a collecting habit, and like hermit crabs, are continually looking for new homes to inhabit. They prefer skulls, but in lieu of these, they will use helmets, large shells, or lacking anything close to this, will create a concrete like husk. This husk is made of their body slime, soil, and other debris. It is not durable, but it makes them very stealthy.
The Ulitka is not a normal creature, they are a peculiar blend of outsider/abomination/undead
The Ulitka has a very unique ability, it can mimic the appearance of any small creature. It generally has to absorb at least one such creature before it can emulate it. The more it consumes of a single species, the better mimic it becomes. It is a not particularly bright member of said species, lacks any innate or special abilities, and it will have abnormal coloration. The Ulitka can never really escape it's dour and unhealthy green-gray patina.
Once you have their tell, you can spot the greasy bastards regardless of what disguise they wear. A seagull, but waxy, with a drooping eye staggering around like its drunk, or the dockyard dog that walks like its got a touch of the dog fever, matted eyes and a runner of drool. They stand out. If you can't kill it in a single blow best to leave it alone. Stick a spear or a knife in it, it can change shape a couple of times, they let out a devil's wail when you stick them, too.
Douse them with salt, if you can afford it, burns em up. Otherwise, fire and oil, or if you have a touch of magic, the cold does em something fierce.
The Ulitka are parasites, hailing from another hellish dimension. There, they are little more than scavengers and composters, carrion eaters. Once brought to the material plane they have no predators, and those predators that run across them are either repulsed by their cold dead-ish flesh, or end up as the Ulitka's next meal.
The Ulitka get in through sloppy summoning spells, poorly done magic gates, and other spells that punch through other dimensions. They rarely show up as adults, instead they seem to propagate through some spore like method. After messy castings of eldritch or necromantic magics, Ulitkas can start showing up.
Slug bois - the presence of Ulitkas can be an indicator that there is a ne'er do well magic user of questionable ability, a sure sign of a diabolist with more ambition than skill, or a low level magus with access to chaos magic, or a powerful artefact that has them pulling more spells than they can handle.
Nightmares - in the city, the number of Ulitkas is increasing, but the locals dont mind. The ulitkas are eating all of the rats, and strays, and stealing trash off the street. Even keeping the beggars and vagrants in check, no one wants to wake up with a slime dog chewing on their face.