Full Toothless cosplayed by Jessica Angus, photo by Enko. I can't imagine how warm this costume must be, but the fabric used really sells the dragon skin look.
Humanized Toothless cosplayed by Professor Awesome, photo by Kassandra. I like this human take on the character with black clothing, wings, leather and plastic claws, and a tail. It definitely seems like a more comfortable option than the full dragon suit.
Classy Toothless dress design by PatyDin. As far as I can tell, no one has made this into a dress yet. It needs to happen. You could top it off with dragon necklace or maybe ears on a headband?
Another complete Toothless suit cosplayed by NeverCryMoon, photo by Alexandra. This suit looks to be made of a velvety fabric, and it's wonderful - I love that this Toothless is paired with a Hiccup.
Go Toothless-casual with this dragon hoodie from Canada Cosplay. The fabric is textured in just the right places, and it has scales and a tail. You could pair it with some black pants for a simple and comfortable cosplay.
Sweatsuit Toothless! Vanessa Brady made her son a Toothless costume by converting a solid-colored sweatsuit. It's an affordable way to get the full dragon look, and she wrote a brief how-to.
Want to see more awesome costumes? I've got a Pinterest board for that. Have any characters you'd like me to feature for "one character, different costumes?" Let me know in the comments!