Right. They probably include 90% of all placental mammals. The groups not included in the boreoeutherians are mostly small, including the Xenarthrans (anteaters, armadillos, and sloths) and the Afrotheres, a hodgepodge of groups originating in Africa, including elephants, sea cows, hyraxes, the aardvark, and some odd critters that used to be classified with the Insectivores.
Notice that carnivores are on the elegant and beautiful side of the family … the Laurasiatherians … primates are all but rodents … the Euarchontoglires … [sticks long narrow snout into the air] … just so we’re clear who’s from the wrong side of the bedsheets …
I appreciate the condescension, but there are two problems.
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Is the animal in question the youngest relative of humans vs horses, or just the youngest ancestor of all placental mammals? I don’t know if horses and humans split off after. Both are mammals, horses are of the order Perissodactyla while humans are of the order primate. But I don’t know if there is a common ancestor before the two orders separated after the first placental mammal showed up.
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There is no mention of mitochondrial DNA in the wikipedia article. It is my understanding mitochondrial DNA can be used to determine when species split apart.
If you had actually bothered to read the link (or the thread, including some of my posts), it’s the common ancestor of Primates and Perissodactyls.
If you don’t want to be condescended to, you should first, y’know, read the thread (and references provided). 
Of course it can, but so can nuclear DNA. I believe the analysis cited in Wikipedia was based on nuclear DNA rather than mtDNA.
Pa-leez.