Yeah, oxen were common for a while as many had been used to pull the wagons themselves on the wagon trains getting there. As were mules. Teams of horses would have also been used to pull those same wagons in the wagon trains, but they were normally multi purpose breeds that could double as saddle horses later rather than true draft horses.
Some (and I emphasize "some") of the settlers to the Great Plains area eventually grew into large commercial farms that would have used teams of horses. But the majority were homesteaders claiming only a single section (1 square mile) of land. After the wagon trek they would have traded their excess horses ( and in some dire cases, eaten them) and retained only the number needed for farming at a subsistence level.
I might add that although many different types of animals were used to pull the wagons in the wagon trains, the trains themselves were usually segregated according to those animals:
Horses and mule only trains with no oxen drawn wagons allowed, and oxen only drawn wagons with no other animal type allowed. Its taught that this was necessary so that all participating wagons could maintain a compatible pace without slowing the others. I have no idea just how true this is though.
