During one of Cosmo's flashbacks it was revealed that as a child he created a giant Hydra, a three headed Dragon, named Snowball who often terrorized Fairy World.
Vicky was also turned into a dragon on in The Fairy Flu, as well as the video game, Breakin' Da Rules.
Another appearance of a dragon was during Timmy's trip to the Dark Ages in Knighty Knight where his parents (who were poofed to the past as well) were abducted by a large pink/light purple Dragon. The Dragon was terrorizing the kingdom of Camelot and eating all its cows. Timmy later got the sword Excalibur to fight the Dragon, but he was forced to fight the Dragon alone as dragons are immune to magic. Timmy was able to do well against the Dragon thanks to the sword. After King Arthur showed up, Timmy gave Arthur his dad's glasses, allowing him to become a strong warrior. Timmy then gives the sword to Arthur who then immediately fights the Dragon. Although Arthur was quickly killed by the beast, he was soon wished back to life by Timmy. Arthur then slayed the Dragon with the legendary sword Excalibur and thus became the new king.
Timmy was turned into a Dragon-like beast during his Fairy Duel with Remy Buxaplenty in Fairy Fairy Quite Contrary which was due to Juandissimo trying to hit Remy with his magic but missing. Since Timmy was the most amazing creature, he won the last round, and thus Remy loses Juandissimo and all memories of him.
Cosmo once summoned a dragon during Truth or Cosmoquences. In 9 Lives!, Cosmo turns himself into a Churro Eating Dragon.