From Middle Chinese 牡馬 (MC muwX maeX).

• (Tokyo) ぼば [bóꜜbà] (Atamadaka – [1])^[1]
• IPA^(key): [bo̞ba̠]

 牡馬 on Japanese Wikipedia

牡(ぼ)馬(ば) • (boba) 

1. male horse
   • 2009, 河原谷創次郎, 天正戦国大戦 二, 学研パブリッシング, →ISBN, page 149:

     前(ぜん)近(きん)代(だい)の日(にほ)本(ん)の軍(ぐん)馬(ば)は、その多(おお)くが牡(ぼ)馬(ば)であったという。

     Zenkindai no Nihon no gunba wa, sono ōku ga boba de atta to iu.

     It is said that the majority of the warhorses in premodern Japan were male horses.

• 種(しゅ)牡(ぼ)馬(ば) (shuboba)

(This etymology is missing or incomplete. Please add to it, or discuss it at the Etymology scriptorium.)

• (Tokyo) おうま [òúmá] (Heiban – [0])^[1]
• (Tokyo) おうま [óꜜùmà] (Atamadaka – [1])^[1]
• IPA^(key): [o̞ɯ̟ma̠]

牡(お)馬(うま) • (ouma) ^{←をうま (wouma)?}

1. male horse
   • 1911, 島崎藤村, 千曲川のスケッチ‎^[2], 青空文庫:

     ファラリイスの駒(こま)三(さん)十(じゅう)四(よん)頭(とう)、牝(め)馬(うま)二(に)百(ひゃく)四(よん)十(じっ)頭(とう)、牡(お)馬(うま)まで合(あわ)せて三(さん)百(びゃく)余(よ)頭(とう)の馬(ば)匹(ひつ)が列(れつ)をつくって通(つう)過(か)したのも、この原(はら)へ通(かよ)う道(みち)だった。

     Fararīsu no koma sanjūyontō, meuma nihyakuyonjittō, ouma made awasete sanbyaku-yotō no bahitsu ga retsu o tsukutte tsūka shita no mo, kono hara e kayou michi datta.

     The road leading to this field was also where a total of more than three hundred horses—including the thirty-four Phalaris foals, the two hundred forty female horses, and the male horses—passed through in file.

From Old Japanese. First attested in the Kojiki of 712 CE.^[2]

牡(お)馬(ま) • (oma) ^{←をま (woma)?}

1. male horse

• 牝馬(ひんば) (hinba, “mare”)
• 騸馬(せんば) (senba, “gelding”)