Perrisodactyls: Their Possible Future in North America 
Perrisodactyls, or odd-toed ungulates, evolved in North America roughly 63 million years ago. Today the group is represented by three families, the EquidaeRhinocerotidae, and Tapiridae. The horses, rhinoceroses, and tapirs respectively. Although rhinoceroses have not been present in North America for roughly five million years, several species of horse were present until the end of the Pleistocene fourteen thousand years ago, as well as five species of tapir, one of which, Baird's tapir, still remains. 
While the reason for the rhinoceros's disappearance in North America is unknown, it is thought, at least by many, that the extinction of the horses and tapirs was anthropogenic, caused by human hunting. With that in mind, I am going to be discussing my current strategy for restoring the ecological niches left empty by these species using introduction of foreign relatives. 
Equids are probably the simplest of these introductions. While there are seven extant species of horse, I have decided to only use three. These three are the takhi (Equus ferus przewalski), onager (Equus hemionus hemionus), and kiang (Equus kiang kiang). This decision stems from a recent idea I had that all megafauna used should be from either Asia or South America, where such animals are at greater risk than in Africa, and could consequently benefit more from range expansion. It is also beneficial to use fewer species to minimize risk of hybridization and competition. My current plan is to use Takhis in all possible areas of Canada, Mexico, and the United States. They will be accompanied by onagers in the United States and Mexico, as well as in the great plains region of southern Alberta and Saskatchewan in Canada. Elsewhere, in Canada and Alaska, the more cold-adapted kiang will be used. Equids are useful ecosystem engineers that create mosaic grasslands ideal for nesting birds and small mammals. 
Tapirs are a little more complicated. Baird's tapir (Tapirus bairdi) is still found in the rainforests of Mexico and Central America, but it's four more northern cousins have disappeared. These were Merriam's tapir (Tapirus merriami), from southwestern US, the California tapir (Tapirus californicusfrom northwestern US, Vero's tapir (Tapirus veroensis) from southeastern US, and Cope's tapir (Tapirus copei)from the northwest US. However it is the opinion of many, including myself, that these may all have been varieties of only one or two species. There are five species of tapir living today which could potentially fill these gaps. My personal opinion is that the mountain tapir (Tapirus pinchaque) should be used primarily, as it is the most climatically suited, as well as being close in size. However it is possible that the Brazilian (Tapirus terrestris), Baird's (Tapirus bairdi), or Malayan tapir(Tapirus indicus) could instead be used in the more tropical and wet areas of the southwest, in areas like Florida, Georgia, and Louisiana. These species are more well-adapted to water, as well as predation by crocodilians. The introduction of tapirs back to North America would bring a new leaf-eating herbivore into the ecosystem, of which North America has few. 
While rhinoceroses have not been native for a very long time, it is possible that they might be introduced as substitutes for other large, thick-skinned animals. An idea I'm entertaining right now is using the indian rhinoceros (Rhinoceros unicornis) as a potential proxy for the North American glyptodont, Glyptotherium. They are of similar weights and diet. 
Anyway, I hope to expand on these ideas later but I want to get some preliminary opinions on what people think.