Possibly a Idiot.
The Slithering Tracker seems like it would be best encountered as a guardian for mad wizard or cult of some kind. A cult of oozes is obvious, but I can see a water based cult doing something like this, in fact, when I first saw the picture I was almost entirely convinced it was an elemental of some kind. The ooze type is one that is very hard guard against. Combine that with something that is not only intelligent, but can also hide in plain sight, and restrain a key member of an attacking force (to the point where their allies will hurt them) makes for a potent bodyguard to anyone who wears robes.
Well, there is more than a bit of misleading involved I would imagine. But if there is one thing I have noticed so far about 5e, it's that it is incredibly easy to give into the dark side, perform or undergo a ritual, and gain lots of power in the span of moments. Given that the average Commoner is effectively CR 0(10xp), and a generic Cultist is CR 1/8, this (and other such transfomations, like the Bodak) is a huge increase in power for any such individual. If you run a game world where being a commoner means being a Commoner (or the like), as contrasted to important NPCs, PCs, and especially monsters, then the average person is massively outclassed in the terms of a fight and their potential impact on the world. A farmer who has just had their family murdered, with no way of striking back at the powers that be, could look on their life thinking they have nothing left to live for. Making this form of vengeance a seemingly viable option, sort of a "Take them down with me!" mentality. Likewise a low level Cultist, who fails to move up in the ranks or garner favors via other means, could be introduced to this idea as an alternate means of service. Which could be more attractive to them than being kicked out of the cult and losing what little status and sense of belonging that they had cultivated, a mindset that often leads people to join cults in the first place.This is the sort of thing that would seem so obviously a bad idea that it makes me wonder just how crap the average D&D world is, if it makes people honestly want to do this.