Depending on who you ask, the Sierra Nevada has two species of deer or two subspecies of deer. In any case, field identification of the Mule Deer versus the Black-Tailed Deer is very difficult and depends on the coloration on the tail and the scent glands on the legs.
Deer are the one large mammal that almost every visitor is going to see. They are nonplussed by the traffic on the village mall and around Cook's Meadow and Ahwahnee Meadow. They're fun to watch and it's shocking how close they will get to people. It is up to YOU to keep the distance safe. The deer of Yosemite are not tame animals and they are unpredictable. Nobody has ever been killed by a black bear, a mountain lion or a coyote in Yosemite, but there has been at least one death in a deer attack and another near death except that the deer was chased off by another person before the attacking deer could finish the job. In this second case, it was a doe attacking with her sharp hooves. So yes, a deer does not need antlers to be dangerous.
Deer, by the way, have antlers, not horns. Antlers are bony structures that grow annually and during the growth period are well-supplied with blood and nerves and a fuzzy skin, which eventually dies and falls away in time to use those antlers during rutting season in the autumn. Yosemite was know, back in the 1930s, as the home to two three-horned deer, thought to have been father and son. The older was known as "Old Horny" despite the fact that he was really "Old Antlery" because he had a single, six-inch long, third "horn" growing straight out of the middle of his forehead.