Pred concept
They're a shape shifter... can come in any form really. An animal, a monster, a person, maybe even someone you know.
They're adept at infiltrating and stalking their prey, taking their time before they strike. And when they do.... The last thing the prey sees is that person melting into a blob of slime before the whole mass lunges and engulfs them.
Maybe it wants energy... maybe it wants nutrients... regardless there's no escape until it deems it appropriate (if ever at all)
If someone were to come across the slime while it was feeding, there's a few ways it could go down. It could spit out their prey and flee, leaving no trace but some slime on the would be victim. The intruder could surprise it and see the prey trapped within this gooey blob creature. Perhaps though, it would shape shift once more, forcing its gelatinous form into whatever fitting appearance. However, it would be unable to alter the bulk of their prey, meaning it would have quite the potbelly if doing so!