My Little Pony is the collective name for small, pastel-colored figurines, easily characterized by their silky, brushable manes and tails and symbol (a picture of groups of pictures) on both sides of their bodies (also referred to as 'rump', 'hips', 'flank', etc.) to match their name. These first six ponies were all Earth ponies; an Earth pony is a little pony with no wings, horn, fins, etc. Each Little Pony came with a ribbon and comb, which became a trait of most ponies who followed. These six ponies all have straight, standard-length manes (falls to or below pony's neck) and tails (tip touches or curls at the ground).

Cotton Candy and Blossom returned in Year 2. And all of the original six ponies were re-issued in 1986 through a special mail offer and renamed Collector Ponies, securing their place in My Little Pony history. When re-issued (in Year 2 and/or as Collector Ponies), each pony was made with design differences (listed below) as well as slightly different coloring in some. Differences I have noted are:
The Collector Pony Blue Belle (shown above) is slightly greenish-blue (mine might be discolored however). Her symbol is darker and her eyes are periwinkle.

The reissued Blossom's (Collector Pony version shown) body color is more of a pinkish-purple rather than the original pale purple. Her eyes are purple.
Butterscotch's body is a slightly darker shade of gold. Her symbol is orange (as opposed to gold) and more defined and her eyes are also gold (lighter), rather than tan.
This version of Cotton Candy shown above is the Collector Pony version. She is a much darker shade pink in body color than the original. The Year 2 version (not shown) is a lighter pink - more salmon colored, but considerably lighter than the later Collector Pony version, closer to the original. Their symbols are pretty much the same, maybe slightly more spread out in the later versions. There is also a version of Cotton Candy (concave version, same hoof markings) with medium blue eyes; her origins are unknown.

The first six ponies are also distinguishable from any other ponies because of their unique hoofs. As shown, the bottoms of their hooves are flat and imprinted with round circles (left). There legs were also shorter and their heads were held lower. However, when these first six ponies were re-released as the Collector Ponies, they were made in the more current pose with their heads held higher, and as pictured, with concave hooves (right). All ponies made in Year 2 and later were made with concave hooves.