Alright, I just "had" to check out the claim of this unkillable fecal monstrosity. Here's some of it's stats: 90hp, Regen 5, SR 50, DR 25/+5. Luckily, it only has one attack and a rather weak one at that.. but still, that's ridiculously nasty for a CR 6 monster.
Here are his 3.5 stats: 140 hp, Regeneration 25, SR 100, DR 25/magic. Immunity to acid, cold, electricity, fire, palarysis, poison, polymorph, sleep, sonic, stunning, all mind effects, critical hits, nonlethal, ability damage and drain, energy drain, fatigue, exhaustion AND TO EVERY NEGATIVE EFFECT KNOWN.
The trick to beat it is with an unknown negative effect. But even if you do kill it "He returns within 1d4 days"