Arctostylopida is an extinct order of placental mammals. They're animals of uncertain affinities to other groups and it was believed that they may be related to ungulates

Ungulate

Ungulates are several groups of mammals, most of which use the tips of their toes, usually hoofed, to sustain their whole body weight while moving. They make up several orders of mammals, of which six to eight survive...

. Originally they were considered to be Northern relatives of Southern American notoungulates

Notoungulata

Notoungulata is an extinct order of hoofed, sometimes heavy bodied mammalian ungulates which inhabited South America during the Paleocene to Pleistocene, living from approximately 57 Ma to 11,000 years ago.-Taxonomy:...

, closer to Notostylopidae. Now it seems more likely they were descendants of Asian Gliriformes, akin to rabbits and rodents. Their tarsal morphology shows moderate similarity to the gliriform Pseudictops, and strong resemblance to the tarsally conservative gliroid Rhombomylus.

Sources

http://www.palaeos.com/Vertebrates/Units/510Ungulatomorpha/510.200.html#Arctostylopida

Missiaen, P., T. Smith, D. Y. Guo, J. I. Bloch, and P. D. Gingerich. 2006. Asian gliriform origin for arctostylopid mammals. Naturwissen-schaften 93:407–411.