You should of planted before the first snow fall actually.
As for what kind of seed, ask your local agricultural rep or even a knowledgeable farmer what they plant.
To get a healthy crop of grass growing that will not be yanked out with the first mouthful you need to leave it for near a entire growing season untouched...say not on it till late August or later so it can establish roots.
That amount of land and 2 horses will not suffice for them grazing grass but a short amount of time, not every day wither.
It will become a dirt field in no time...best plan on having hay to feed them all of their meals.