Slime monsters are almost a given in any fantasy RPG, but Dark Souls' other underground locations.
In addition to not being nearly as deep as players may think at first, Dark Souls' Depths hide some pretty nasty secrets. Players will face off against Hollows, Giant Rats, Undead Attack Dogs, and some of Dark Souls' most annoying creatures, Basilisks. However, it's not the deadliness of enemies like the oversized Hollow Maneater Butcher which makes The Depths so unnerving, but the implications they - along with Dark Souls' slimes - create simply by existing.
In the Dark Souls Remastered Collector's Edition Guide published by Future Press, a piece of Dark Souls concept art for the game's slime monsters is included, and the book states the artwork's title translates to "squirming rotten meat." It seems the slimes in Dark Souls, although somewhat glossier in their in-game version than in the original concept art, were intended to be made of living Undead flesh.
Dark Souls' Undead Cannibal Cycle
Above where Dark Souls' slime monsters are first encountered is where the Hollow Maneater Butcher enemy is located, a character who is seemingly content chopping up meat until it is disturbed by the player. As the Dark Souls Remastered Collector's Edition Guide notes, "if one Undead eats another Undead, the waste and remains don't just die - they can't!" This points to Dark Souls' slimes not just being another RPG trope, but actually, as the book puts it, "reanimated Undead waste or possible the offal from the butchering of an Undead."
It is likely that Undead either fall in battle or are captured by other Hollows, and are then brought to the butchers in Dark Souls' Depths. There they are chopped up into food and, some time later, being to regain some sort of control over their movements despite being either digested or dismembered. Some resemblance, however small, of life seems to remain in them, as players like @illusorywall have noticed certain slimes appear to have a face.
This is an entirely disgusting way for an enemy to be created, but it makes sense for Dark Souls, especially since the very next boss players encounter after discovering the game's slimes is the Gaping Dragon, which is practically turned inside-out. There are no shortage to horrifying creatures and disturbing stories in Dark Souls, but the way the game's slime enemies are created might be one of the grossest.
Source: Dark Souls Remastered Collector's Edition Guide
Image Source: @Illusorywall/Twitter