The first thing I want to say is that I am NOT an expert on the Petite Ponies, nor do I really collect them unless one comes up and I'm in the right mood to buy it :)~
Therefore THE best resource for Petite Pony research can be found at the Royal Fortress's Petite Pony Guide.
As a brief overview, however, Petite Ponies were a weird experiment by Hasbro into the world of 'Honey I Shrunk The Ponies!'. About the size of a Mummy Charm, but with a horseshoe hoofprint, these little characters were first discovered in the Pony Comic by two of the Newborn ponies, and would make spasmodic appearances as time went on.
Hasbro made various playsets to go with these little guys, including a Royal Castle, a Carousel and various shops and houses. For some unknown reason, however, only the ponies sold with these playsets were ever given names. All the subsequent sets of Petite Ponies were not given their own identities.
Poor mites :)
Above are the array of ponies sold with playsets. Please note that one (Misty) is absent because I just don't have her. Please also note that there exist versions of each of these with MOULDED tails. However ALL playsets sold in the UK came with the rooted tail versions shown above, even though Topper was given in the comic as an Earth Pony. Princess Moondust and Kingsley had different names in the US edition. I know Princess Moondust was Magic Star...but I'm not sure on Kingsley.
And here is a selection of the other Petites on offer. Please note there were also Glow & Show Petites and petites with moulded tails, but I don't have any of the former, and the latter I only have MIB somewhere beneath my Jem dolls :S I told you this wasn't my area of expertise.
The Mummy Charm is included for comparison.
Many of these sets seem hard to find second hand - the Ponytail Petites are the exception, as they seem to be the most common of the Petite Pony sets.