Of course it's a fart. Morrowind is a very serious game.
Hmm.. The fetid green gas bursts come out without the characteristic fart sound though. I guess that would be just too obvious.
what I want to know, is what do they eat, and where are their mouths? Given that they have down-facing tendrils, I'd assume the mouth is in the middle of them, but alas-- Nope.
(Also, if Bull Netch are so lazy, could not one just tie a bunch of them together with netting, and ride on their backs? If they are as lazy as MK suggests, they wouldnt resist the process, or the bonds, as long as they had food under them.)