Onesies are very popular in fashion now. It's an odd concept, yes, but it's one that just seems to work. It's one of those trends that come to the fashion world, makes a big scene and then settles down.
The trend came about from a brand called OnePiece and I first came across onesies a few years ago at the Clothes Show. How? A lot of really hot people were wearing them... That's how you get attention. From a marketing point of view, that was a really good move.
Anyway, pretty much all the major high street brands and some higher fashion brands have their own collection of onesies now and I don't think they'll go away any time soon. It's not because they're fashionable. They can be, but most people, in my opinion, have them because they're warm, comfortable and just a bit funny.
I have two onesies. One is from Primark, from one of my close friends, and it is a cat onesie; the other one I bought myself and it is white with bright pink hearts all over it from New Look.