You could try keying it. Obviously there is a lot of green around the horse. Of course you will end up with noisy areas, but I tend to believe that those could be fixed with additional effects treatments using Minimax, Matte Choker and other tools. You still will need to do some masking, though. Other than that you could attempt "smart" tools like the motion keyer in Boris Continuum or Power Matte, but obviously this will add to your budget, so masking is probably way to go. It's a mere 10 seconds, so I don't think it's that bad after all...
From looking at the low rez preview I would think you would need to do this in steps using different techniques on multiple layers to build up a high contrast black and white image you can use as a track matte.
Start by using the mask tools to create a garbage matte. IOW, create a mask as close to the horse as possible. This removes most of the background.
Now duplicate this layer a couple of times and start with the top one.
Use a Keylight or maybe Color Key to try and generate a fairly decent matte. If you are using Keylight set it to view final matte. You want to create a black and white image.
Then turn off the visibility of the top copy and start working your way down to create additional black and white layers that you can blend together to create a clean black and white matte. Try adjusting channels, try a second keyer, then experiment with the blend modes using screen to lighten gray areas and multiply to darken them until you get a good clean matte.
Now pre-compose these layers and drop the original footage below the pre-comp and set the pre-comp as a luma matte for the footage. If you end up with a bunch of green spill light from the grass around the edge of the horse you can almost instantly fix that by adding Keylight to the bottom copy of the layer, setting the key color to green. If you have holes appear in your horse you can tweak Keylight so that they go away. In this case you're only using the keyer to fix the color spill, the track matte is cutting out the image.
I hope this helps. In most cases you'll have much better results and get where you want to go faster if you don't try to make every correction on one layer.
I hope this makes sense. Let me know if you have any questions.