There are many variants available for each of the Skylanders games. These variants rarely receive an official announcement so it's up to the fans to find them, name them, and let their presence be known.
The majority of Skylanders variants are inserted into random boxes that are shipped to retail stores. No one knows exactly how many of each variant exist and you never know where they'll end up.
Occasionally there will be special promotions through which you can obtain some variants for a limited time (i.e. Sidekicks, Flocked Stump Smash).
Finally, a few, select variants are handed out at industry trade shows like E3 and Toy Fair.
If you want to know where to find these variants or more about just how rare they are, be sure to check out the “How Rare Is My Skylander?” series:
Part 0 – Other Notable and Hard-to-Find Skylanders
Part 1 – Spyro's Adventure Variants
Part 2 – Giants Variants
Part 3 – SWAP Force Variants
Part 4 – Trap Team Variants
Click the pictures for more information and a larger image.