I also had some problems with the movie -- but if I was willing to bend over backward, I could explain the horses a few ways:
1) The spaceship contained horse embryos as part of a planet-seeding starter kit.
2) After conquering Earth sometime in the past, General Thade travelled back to his home planet and planted horses there himself. Along with a time capsule for Colonel Attar containing an elocution manual.
3) The humans that didn't evolve into supermodels and country singers devolved into horses.
I cannot, however, explain why Rick Baker didn't pick up another Oscar for makeup.
Horsies
1) The spaceship contained horse embryos as part of a planet-seeding starter kit.
2) After conquering Earth sometime in the past, General Thade travelled back to his home planet and planted horses there himself. Along with a time capsule for Colonel Attar containing an elocution manual.
3) The humans that didn't evolve into supermodels and country singers devolved into horses.
I cannot, however, explain why Rick Baker didn't pick up another Oscar for makeup.