Actually...yeah, I think I'm going to have to go with the monkey now. I don't like the idea anywhere near as much as the tiger (or any other type of cat), in all honesty, but having read through some more tweets, it seems the more likely candidate.
Here's the mob isolated from the grass, and with perspective adjustment.
The monkey suggestions in the OP are ignoring some artifacts in this image, like the one left of the "face", and the top thing.
And here's it with a grid so we can compare the dimensions. We must consider that a mob's texture uses a pixelated image, so every different color must have it's own square (pixel), or groupd of squares (pixels).
Considering that the mob would have a standart 8x8 pixels face, this is what we would have, based on the picture (and using the same colors of course):
Looks more like a feline (tiger, leopard etc) to me than a monkey.
No monkey has multiple colors in the side of the face, or in it's paw/arm, like the above analisys shows us.
I give you the face of the new mob that I drew from the screenshot
Looks like a monkey to me. Kind of a dull face with lowered eyes much like the wolf in its untamed state. If you look at the bottom right image which is blown up a little, you'll see what might look like a paw.
Monkeys were also tweeted as a possibility by Jon, it has to be a monkey.
The one weird thing though is that if you look at the side of its head in the actual screenshot, there is no ear on the side but it looks like there is one on the top which is a bit confusing.
Edit 2: If you look to the left side of the tall grass and behind the dirt block that it is sitting on, there is a leaf block there, meaning obviously that it is a bush. The leaf block probably keeps running over under the mob behind the tall grass which means that the mob is most likely 1x1 block
New mob is small.
Monkeys are small.
Therefore, new mob is a monkey.
That's my take on it
Edit 3: Picture of model provided by Repeatist
If the model is real, I guess we won't really know until tomorrow but it seems completely plausible
It turned out that the image I was given was just a clever manipulation. I do apologize if I misled most of you, but you have to understand that I had hoped for a monkey mob just as much as the rest of you. Oh well, I suppose a tamable ocelot wouldn't do so bad for the meantime. There is still hope for tomorrow, after all.