I'm trying to put together some documentation on the relation between the common (modern) horse and the okapi. I've found a tree that shows that connection but it has no references. I'm hoping that someone has documented this connection somehow.
tolweb.org is your friend!
Because the okapi and the horse are not so closely related you will probably find several articles on the subject. Okapi belongs to the order of artiodactyla while the horse belongs to the order of perissodactyla. They are both mammals, eutheria, ungalata (superorder).
These two references may help you (I only read the titles)
Graur, D., M. Gouy, and L. Duret. 1997. Evolutionary affinities of the order Perissodactyla and the phylogenetic status of the superordinal taxa Ungulata and Altungulata. Molecular Phylogenetics and Evolution 7:195-200
Xu, X. F., A. Janke, and U. Arnason. 1996. The complete mitochondrial DNA sequence of the greater Indian rhinoceros, Rhinoceros unicornis, and the phylogenetic relationship among Carnivora, Perissodactyla, and Artiodactyla (plus Cetacea). Molecular Biology and Evolution 13:1167-1173