Board Thread:Naruto Discussions/@comment-26084508-20150916103812/@comment-17623-20150916130352

The belief that their strength is based on the number of tails seems to really only be held by Kurama, something Gyuki didn't necessarily agree with, and somewhat demonstrated considering the Eight-Tail should have been able to easily over power Two through Seven otherwise.

Realistically, there isn't a lot o evidence, when the series began, that Kishimoto thought of having more than two, Shukaku and Kurama. In fact, as I can gather there really wasn't any talk of there being tailed beasts at all until Part II. So it was probably more or less Kishimoto just deciding, hey I can run with this difference number of tails thing.