Does documentation exist that shows the evolutionary connection between common horses and the okapi?
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).

Here is the tree of the eutheria (within the mammals) from tolweb. You have approximatively 150 references. You can click on perissodactyla and artiodactyla and you will find 100 more references.

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