I think it started as a private joke between a couple of furs. Then they made icons and others followed suit.

Somethin' like that.

haha, if memory serves me right, there's a little snippet on Family Guy where this dude confesses to his gf that he's "actually, a woman" then removes a mask to reveal a woman! then it happens again, "actually, I'm a horse" and then, in an inexplicable way, "actually, I'm a broom." then the broom stick falls over onto the ground.

here's a crappy video of it :3
http://www.youtube.com/watch?v=LvkTdyXVh3U

This may or may not be what this meme on here is referring to, but it was the fist thing I thought of when I saw a few of them XD

I believe the meme here started when someone made their icon a picture of their own face with "Actually, I'm a Human" on it.