Winter Wolves were selected specifically because they were a little outside the norm. In addition, Mongoose wanted their first PDF "Mini-Slayer's Guide" to be about half the size of a normal Slayer's Guide, so it was necessary to choose a monster that probably wouldn't "fill up" a full-sized Slayer's Guide. On the other hand, winter wolves are intelligent creatures that have their own society and culture, and are thus within the spectrum of creatures that a Slayer's Guide could focus on.
But, not to hijack this thread any further, the 2nd Edition Monstrous Manual states:
Myconids live in Underdark regions...Myconids try to find isolated spots away from civilized areas. These communities will usually be near water, for they like dampness.
I'd be willing to bet that they don't really need to breathe (they certainly don't need air to speak aloud, as they communicate solely through rapport spores), and thus could function perfectly well underwater for extended periods of time. But that's just my opinion.
And until the myconid becomes Open Game Content, I doubt that we'll ever see a Slayer's Guide to Myconids. Maybe we'll be lucky and the little fungus-men will show up in the Monster Manual II. (I'm assuming that will eventually become Open Game Content; has anyone heard one way or the other?)
Johnathan M. Richards
Author, "The Mini-Slayer's Guide to Winter Wolves"