A flail is basically a stick with a pointy ball chained to one end. It does one attack per turn.
A dual flail attacks twice (it has two balls).
Now replace each ball by an octopus. And each octopi is holding a cat on each of its 8 tentacles. So when you attack, the cephalopods attack, and that means that 16 angry felines attack. I think at the time they came up with this animals had some sort of guaranteed damage exception in some cases (perhaps in a previous DND version?).
Anyway it was completely OP.