Nilbog always spawns his pool between himself and me (at a certain distance from him)...

Slink always spawns pools under himself and the players...

Breeze puts his pool under the enemy...

You can even predict where Ulfang and Uberman are when they appear, as thy always spawn alongside the mobs you cleared a few seconds prior...

I've been boggling my mind trying to understand how Shady and Surge work but it seems so random @.@