Following my apparent mantra of Portrait discussions I have this to say.
In the previous PTR patch this portrait was added to the game:
The portrait used the Zealot Chef as a placeholder.
Now that the patch has gone live the portrait spot has remained:
But now it has a solid blue square as its placeholder.
I think it's pretty obvious it's a placeholder for the Blizzard Dota portrait reward (each of their mods had a portrait you could gain by earning an achievement) that was tested in the PTR and accidently made it to the game with the latest patch. It will probably be removed with the next patch.
And for those of you who think it is a reward you get if you get all the achievements, this man has all the achievements and does not have it unlocked. When hovering over the achievnet it does not give u the corresponding achievement like the other portraits do.