Christophe Courteau / Male BurchellÕs zebra (Equus quagga burchellii) pulling the head of a dead pregnant female that died due to complications whilst giving birth, trying to wake her up, Etosha National Park, Namibia. Par...

Carol Walker / Two wild pinto Mustang stallions battle for dominance in Sand Wash Basin, Colorado, USA.

Carol Walker / Head portrait of grey Arabian stallion running in desert dunes near Dubai, United Arab Emirates.

Carol Walker / Two wild Mustang stallions fighting in the White Mountain Herd Area, Wyoming, USA. August.

Kristel Richard / Headshot of two rare black Kladruber stallions, harnessed for driving, at the Great Riding festival, in Slatinany national stud, Pardubice Region, Czech Republic. June 2015.

Kristel Richard / Two wild Przewalski / Takhi Horse (Equus ferus przewalskii) stallions play fighting Hustai National Park, Tuv Province, Mongolia. Endangered species. September.

Kristel Richard / Przewalski / Takhi Horse (Equus ferus przewalskii) breeding stallion stands alert, Hustai National Park, Tuv Province, Mongolia. Endangered species. September.

Richard Du Toit / Burchell's Zebra (Equus burchellii) stallions fighting, Rietvlei Nature Reserve, South Africa

Richard Du Toit / Burchell's Zebra (Equus burchellii) stallions fighting, Rietvlei Nature Reserve, South Africa

Yva Momatiuk & John Eastcott / Wild Horse (Equus caballus) stallions play fighting, Pryor Mountain Wild Horse Range, Montana

Donald M. Jones / Wild Horse (Equus caballus) fighting stallions, Wyoming

Kristel Richard / Two Cambodian Pony stallions play fighting, rearing on back legs, Siem Reap, Cambodia. March 2013