They however are *quite* stunnable. Tishan's Discord / Marker's Clash / Ice Comet ...and don't forget to wear your diamondskin absorb skin and drink a potion of SOW.
OK...so the Wildings are supposedly immune to the stun effect - but it stunned anyway. And they go down with fire just as fast as anything else. The guardians are another story..stun em, but do not use fire.
Though it may be true that the Potameids in general cannot be snared, rooted, or slowed, I can attest to the fact that they can, indeed, be charmed and mezed. As a bard I practiced Charm kiting there, as I can usually pull only a couple at a time. It's the turtles that bug me though, but that's for a different post...
Cannot be snared, darkness won't land, can't be rooted either. Massive fire resistance. However, disease and poison land quite nicely. Don't know if they can be mezzed or charmed. My group had me (47th Necro) a 51st Warrior and a 54th Cleric. One Potameid was easy, two were a handful, and three resulted in the death of the cleric.