- Arapahovius advena Bown & Rose, 1991 †
- Arapahovius gazini Savage & Waters, 1978 †
Mammalia → Primates → Tarsiiformes → Omomyidae → Anaptomorphinae → Anaptomorphini
, Arborimus
- †Archaeolagus Dice 1917
- †Hypolagus Dice 1917
- †Notolagus Wilson 1938
- †Panolax Cope 1874
Mammalia → Lagomorpha → Leporidae
- †Archaeolagus acaricolus Dawson 1958
- †Archaeolagus buangulus Stevens et al. 1969
- †Archaeolagus emeraldensis Barnosky 1986
- †Archaeolagus macrocephalus Matthew 1907
- †Archaeolagus primigenius Matthew 1907
Mammalia → Lagomorpha → Leporidae → Archaeolaginae
, Archboldomys
- Menotyphla Haeckel
Mammalia → Eutheria → Placentalia → Euarchontoglires
, Arctictis
, Arctocebus
, Arctocephalus
, Arctogalidia
, Arctonyx
, Ardops
- Ariteus flavescens (Gray, 1831)
Mammalia → Chiroptera → Phyllostomidae → Stenodermatinae → Stenodermatini
, Artibeus
- Antilocapridae Gray, 1866
- Bovidae Gray, 1821
- Camelidae Gray, 1821
- Cervidae Goldfuss, 1820
- Giraffidae Gray, 1821
- Hippopotamidae Gray, 1821
- Moschidae Gray, 1821
- Suidae Gray, 1821
- Tayassuidae Palmer, 1897
- Tragulidae Milne-Edwards, 1864
Mammalia → Theria → Eutheria
, Arvicanthis
, Arvicola