For a small pond such as this, your best choice would be a couple of the smaller carp: shubunkins or comets. Shubunkins are calico fish (tri-colour in white, red, and black), while comets are generally considered miniature koi in colouration. They require at least 10 gallons of space per fish, so you can easily calculate the proper number for your pond.
While the fish will happily eat mosquito larvae, you should also provide them with food each day. Pellet type fish food is better than flake fish food for ponds as it makes less of a mess in the pond.
If you are including fish, you will need a filter for them. No fish should be placed into a tank or pond without a filter! Choose a filter that is appropriate to the size of your pond. I use a double filter (that is, twice what would normally be required by that size pond) on any pond that contains carp as they are "messy" and will dirty the water quickly.