actually, its more 1/16000... i'm still trying to find the numbers... :S...
They come in groups of 5.
You can check the pages I have made for the Pinkpuff

Zyrthofar's Pinkpuff page

Aaaaah... recalculated everything at last... here goes...
I still haven't double-checked the odds of encountering the Pinkpuff groups (is my memory's good, that's 1/64 for getting the rarest enemy group) and one of them dropping a pink tail is 1/16384.

Each pinkpuff has 5% chances of dropping an item, and this item has 1/64 chances of being a pinktail. (that means 1/1280 chances of dropping a pinktail). There are 5 pinkpuffs, so 5/1280 = 1/256... the odds of getting the pinkpuff group is 1/64, x 1/256, giving an average of 16384 encounters in that small room in order to get a pinktail.

Hope you follow me...

Btw, the Adamant armor really, really is worth the trouble.
Zyrthofar's Adamant Armor Page

You can think of this as levelling-up you can hope to leave the room with a pink tail at level 80+.