Dragon

The first dragon appearing in EGS was the Taurcanis Draco summon in New and Old Flames. However, later there were signs that it's not the only dragon in EGS - there was one in Pandora's backstory, Magus mentioned his universe has dragon sirens, dragons being as common a danger as tornadoes, and there was a panel full of smoke which featured Ellen in a vaguely dragonish (or maybe half-dragon?) form.

Arthur mentioned the core of Dewitchery Diamond (previously assumed to be wooden by Magus) actually being scale from a very large animal. Ellen didn't have all the information we have, but she knew about Dame Tara and Andrea and Magus' dragons and is a champion in conclusion jumping, so immediately guessed it was a dragon scale and that both her and Ashley are now infused with dragon power.

The curses that the Dewitchery Diamond was created to cure, but ended up spreading, were generated by a powerful and ancient Immortal who presented himself in the form of a dragon.

Arthur immediately denied the existence of true dragons - not counting the Taurcanis Draco as a true dragon, because it's merely dragon-like, because it's a summon or both - but he has been wrong before.

Outside continuity

The Magickal Gatherings card Literary Dragon

The movie Sarah, Elliot, Tedd, and Grace watch in Night Out features a dragon. The dragon behind Lucy in the EGS Main Story Comic for 2013-06-05 is part of the background of her avatar. Dan rides a dragon in the EGS Sketchbook Comic for 2007-02-18. There are multiple dragon-related cards in Magickal Gatherings like "Literary Dragon", "Big Pointy Teeth Drake", and "This Thing".

Grace rides a dragon in the EGS Sketchbook Comic for 2011-11-30. The Writer's Block fights a bunny dragon in the EGS:NP Comic for 2015-07-13. Justin is drawn as a draconic pirate in the EGS Sketchbook Comic for 2014-09-12.