It always bothered my overly analytic mind trying to understand WHY Tails had two tails. Honestly speaking, Sonic was a kids' game, and as such, most kids, at least ones I knew, didn't care about such things. But since I'm a nerd I'm going to give my possible explanation to Tails' genetic malformation.
And just for fun, a dumb running sonic I animated. This is probably what happened to Sonic after finding out about Tails and his twin.