Curb Your Dragon (Season 1, Episode 8)

Alex buys Justin an enchanted dragon disguised as a Beagle, because she feels bad she lost his other dog 9 years ago. However, since it can still breathe fire and fly they have to hide from their parents that the dog is a dragon saying it is a lost dog and hanging up posters. One day, while they are at school, the dragon seller comes back, claims the dog and enters it in the local dog show where Alex, Justin, and Max must outsmart the dragon seller and retrieve Dragon.