While this animal might look like some sort of deer or horse, it was actually only distantly related to any modern hoofed mammals.
This is Thoatherium from the Early Miocene (~17-16 mya) of Argentina. About 70cm long (2′3″), it was related to the weird llama-like Macrauchenia and was part of an extinct group of ungulates (the Meridiungulata) which evolved during South America’s time as an isolated island continent.
It was adapted for fast running, with long legs and only a single horse-like hoof on each foot – but it was even more one-toed than modern horses are, having no remaining “splint bones” from vestigial side toes.
Homalodotherium, a South American notoungulate mammal from the Early-to-Middle Miocene of Patagonia (~20-15 mya). Standing about 1.4m tall at the shoulder (4′7″), it seems to have convergently evolved to fill the same selective browsing niche as the North American chalicotheres and the later giant ground sloths.
Despite being an ungulate it had claws rather than hooves, and walked plantigrade on its hind feet but digitigrade on its front feet. It would have been capable of rearing up bipedally to pull down branches with its long forelimbs, with the shape of its nasal bones suggesting it may have also had a prehensile upper lip to help it strip off vegetation while feeding.