Common or Garden Dragons, also spelt as Common-or-Garden Dragons, are the most common dragon (as the name suggests) and can be found in most dragon caves. In the first book, Hiccup finds a Common or Garden Dragon while looking for a dragon to tame at Wild Dragon Cliff after having given his Basic Brown to Fishlegs. Hiccup claims he could 'sense' the Common or Garden, since it was too dark to see it. Hiccup later names it Toothless.
Common or Gardens are said to be the size of a West Highland Terrier (aka Westie).
While the Common or Garden Dragon from the book doesn't actually appear in the film, the creators developed a dragon to pay homage to the Common or Garden Dragon (especically Toothless) called the Terrible Terror.