You will be able to have multiple pregnancies over the duration of the game. It will be possible to have twins, triplets etc (small chance) but not multiple pregnancies simultaneously. There are always special items like birth control or abortion pills if you decide you want Tiffany or the other girls to be bred by different creatures/enemies.
Right now it's only possible to be bred by humans, but there will be something that happens during the main plot that will make it possible for interspecies and undead breeding
So yes, eventually she can be bred by horses.