To be utterly and completely confusing, the Sparkle Ponies saw two seperate releases - a Mail Order and a bog standard store release. Although the mail order issue never came out in the United Kingdom, there have been reports of Mail Order ones sealed on card so a few did make their way over here in random amounts.
This section deals with the versions that were sold in stores. For most of the ponies, the main difference between mail order and store sold was the symbol on the cheek (or lack of), though there were different shades of plastic and most significantly, two ponies with different hair colours.
To find out about the Mail Order Sparkle Ponies, visit this page.
Store sold Sparkle Ponies did NOT have symbols on their cheeks.
The Sparkle Ponies were a brand new kind of My Little ony gimmick. They featured translucent plastic, dotted through with specks of glitter and, true to their name, each also featured tinsel in her hair - a style up till now only used by royal ponies.
The UK pony comic introduced the Sparkle Ponies in comic edition 100, with Star Hopper, and one of the many explanations I remember for their sparkles is that they were exposed to sparkles from Sky Rocket's special fireworks which gave them their unusual appearances. This may very likely have been Sky Rocket's backcard story, since it was the only one I had as a child (but I no longer have the card to verify that).
The set featured one unicorn, one pegasus and four earth ponies, all of which were released globally. Their packaging in the US and the UK was actually very similar - the only distinct difference being the colour cloud in which the pony's name was written (in the UK it was yellow, whilst I believe in the US release it was pink).