I say canine. But you do see a lot of fan art though (mostly anime [Naruto for example])...
Depends on who you watch I recon.

Fanart

Except fan arts-wolfies. Scene dogs, sparkle dogs. Wolfies wolfies and again wolfies. Ppl love them -_-

Fanart - specifically for popular/mainstream animes/mangas/comics/etc.

canines and humans

I believe that so far (on deviantart anyway) it's a pretty much neck'n'neck between feline and canine art. Although there have been a lot of scene/emo/sparkle dogs or cats or whatever, that are so mutated they could just as easily be a fruit bat or an aardvark. Just and observation. Many people also seem to be creating there own species lately which is actually pretty cool.

Internet art at this date and time is a tie between canine and feline.

Professional art is fantasy, animal and humanoid. (e.g the games Magic TG, D&D, etc)

Art everywhere and any time I'd say humans, because old art (pottery and such) usually depicts humans and humanoids. There is probably more human art in the world then anything else.

It's true that the internet in today's society is becoming overwhelmed and in some cases, obsessed with the amount of cat or cat-related comedy on the net you find. Hell most every party I go to, at some point ends up with a group of drunken mates watching cats fail on youtube.

But in my opinion, here on DA, what is most popular is Fan Art, or fan based art inspired by other artists, styles, and media like movies ESPECIALLY Disney.
There is ten to the millionth Lion King fans drawing on here as well as 'Pony Art', and Marvel type drawings. Fan Art is big and ever-growing since all these things have made great impact on our lives like The Lion King, like Marvel and DC comics, the movies, the books, the famous artists inspiring today's society into Indie trends and Gaming, how popular is Nintendo's loving Yoshi, Mario, and Toady?

Fan Art....You may not know it, but it's there more than we let on or like to think.

On DeviantART, it's canines, no question, followed by felines. Universally, women. Definitely women.

I think there is probably more dragon fantasy art, but no one is looking for it.