50 50 for me Certainly most of the capitalization sources for Mustang are from specialist horse organizations They probably should carry more weight than your average newspaper But most average English sources would spell it mustang not that Wikipedia really goes with what most English sources use per my observations Whatever the sourcing Wikipedia goes with consensus But when an Rfc gets plopped up and a mere 13 hours later on a weekend an editor calls snow and reverts the article when 5 of 7 posters see things as he does that is disruptive editing There may be many equine editors who haven t even seen this yet Were the Equine Livestock and United States projects notified since this article lists them as categories If not I can do it to make sure they have a chance to comment The thing that bothers me most about de capitalizing this are that breeds are capitalized here A Mustang may not meet official breed nomenclature because of the feral wild nature they tend to be treated as breed like in many writings and our readers will scratch their heads as to why I know I do Why Quarter Horse or Thoroughbred should be always be capitalized here yet mustang should not is quite strange and arbitrary to me I would tend to lean in favor of capitalizing for that fact alone Mustangs are kind of a special case it s not like mountains will topple if it s left capitalized like other horses breeds are