Opinions from A. -M. Forsten 1975
- 'Anchippus is a subjective synonym of Parahippus'
- 'Anchippus texanus is recombined as Parahippus texanus'
- 'Anchitheridae was reranked as the subfamily Anchitheriinae and belongs to Equidae'
- 'Anchitherium belongs to Anchitheriinae'
- 'Anchitherium (Hypohippus) was reranked as Hypohippus and belongs to Anchitheriinae'
- 'Anchitherium (Hypohippus) affinis is recombined as Hypohippus affinis'
- 'Anchitherium (Parahippus) was reranked as Parahippus and belongs to Anchitheriinae'
- 'Anchitherium australis is a subjective synonym of Hypohippus affinis'
- 'Anchitherium clarencei is a subjective synonym of Anchitherium navasotae'
- 'Archaeohippus belongs to Anchitheriinae'
- 'Archaeohippus nanus is a subjective synonym of Archaeohippus blackbergi'
- 'Astrohippus curtivallis is recombined as Pseudhipparion curtivallum'
- 'Calippus anatinus is a subjective synonym of Calippus regulus'
- 'Calippus martini is recombined as Pliohippus (Astrohippus) martini'
- 'Calippus optimus is recombined as Pseudhipparion optimum'
- 'Calippus regulus belongs to Calippus'
- 'Dinohippus subvenus is a subjective synonym of Pliohippus supremus'
- 'Eoequus is a subjective synonym of Merychippus'
- 'Eoequus wilsoni is a subjective synonym of Merychippus circulus'
- 'Equidae belongs to Hippomorpha'
- 'Equinae belongs to Equidae'
- 'Equus (Protohippus) perditus is recombined as Merychippus perditus'
- 'Equus laparensis is a subjective synonym of Pliohippus martini'
- 'Griphippus is a subjective synonym of Pseudhipparion'
- 'Hipparion occidentale is recombined as Neohipparion occidentale'
- 'Hippomorpha belongs to Perissodactyla'
- 'Hippotherium ingenuum is recombined as Nannippus ingenuum'
- 'Hippotherium ingenuum is recombined as Nannippus ingenuus' according to Quinn 1955
- 'Hippotigris parastylus is a subjective synonym of Pliohippus martini'
- 'Hippotigris sellardsi is a subjective synonym of Pliohippus martini'
- 'Hippotigris sellardsi is recombined as Astrohippus sellardsi' according to Webb 1969
- 'Merychippus belongs to Equinae'
- 'Merychippus francisi is recombined as Calippus francisi'
- 'Merychippus gunteri belongs to Merychippus'
- 'Merychippus gunteri is a subjective synonym of Parahippus leonensis' according to Bader 1956
- 'Merychippus socius is a subjective synonym of Parahippus leonensis'
- 'Merychippus vellicans is a subjective synonym of Parahippus leonensis'
- 'Miohippus blackbergi is recombined as Archaeohippus blackbergi'
- 'Miohippus blackbergi is recombined as Archaeohippus blackbergi' according to Bader 1956
- 'Miohippus navasotae is recombined as Anchitherium navasotae'
- 'Neohipparion belongs to Equidae'
- 'Neohipparion belongs to Equidae' according to Webb 1969
- 'Parahippus barbouri is a subjective synonym of Parahippus leonensis' according to Bader 1956
- 'Parahippus barbouri is a subjective synonym of Parahippus leonensis'
- 'Parahippus leonensis belongs to Parahippus'
- 'Parahippus minutalis is a subjective synonym of Archaeohippus blackbergi'
- 'Pliohippus belongs to Equinae'
- 'Pliohippus (Astrohippus) belongs to Pliohippus'
- 'Pliohippus (Pliohippus) belongs to Pliohippus'
- 'Pliohippus circulus is recombined as Merychippus circulus'
- 'Protohippus (Calippus) was reranked as Calippus and belongs to Equinae'
- 'Protohippus lenticularis is recombined as Nannippus lenticulare'
- 'Protohippus supremus is recombined as Pliohippus (Pliohippus) supremus'
- 'Protohippus vetus is recombined as Merychippus vetus'