Some sources like the 36 Lessons keep implying that the Hist is if not the same thing as Sithis then at least related. Repeated references of calling Sithis SITHIST which is just Sithis and Hist mixed.

Shit-Hist? Pretty sure that's supposed to be Sith-Hist. Still, a very interesting point.

Dammit! I did that before and had to edit 

According to Olik-Jaa, it's a haunted bog peopled entirely by his ugly frog-faced cousins, the "Ghost People." Apparently, they eat poison toad-hearts so that they can raise the dead or some such nonsense. Oh, and they can also turn invisible and talk to tree roots. I honestly wish I was a storyteller sometimes. The things these lizards come up with really are beyond belief. On The Ghost People

I believe he mentions a tribe of Naga, they are a subspecies of Argonians (hence the cousins) and in the case of the ghost people, their skin is translucent and they offer corpses as compost to their hist. However unlike this quote, they are often described as being similar to snakes, frogs or even cuddlefish, not just as frogs.