The hydra flail seems to be the same in appearance but the mechanics are a bit different. The information given in BoVD is as follows:
Yeenoghu wields a Huge +5 triple flail. This is a
unique exotic weapon. Each time a hit is scored, roll 1d3 to
see how many of the heads hit the target. Each head deals
1d12+24 points of damage. If more than one head hits the
same target, the victim must make a Fortitude save (DC 17) or
be paralyzed for 2d6 rounds. If all three hit, the target must
also make a Will save (DC 17) or be confused for 10 rounds
as I see it according to the table for decreasing damage by size in the DMG each head should do a D8 for a medium weapon. My questions are how many hands does it take? It never says. How much does it cost? How does enchanting work? it isn't technically a double weapon like a dire flail but the heads hit separately so should you have/get to enchant each head separately?