Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) divided the spoils, allotting two shares to the horse and one share to the man.

عن عبد الله بن عمر رضي الله عنهما «أَنَّ رَسُولَ الله صلى الله عليه وسلم قَسَمَ فِي النَّفَلِ: لِلْفَرَسِ سَهْمَيْنِ، وَلِلرَّجُلِ سَهْمًا».
[صحيح] - [متفق عليه]

‘Abdullāh ibn ‘Umar (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) divided the spoils, allotting two shares to the horse and one share to the man.
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

‘Abdullāh ibn ‘Umar (may Allah be pleased with him) tells that the Prophet (may Allah's peace and blessings be upon him) divided the spoils, allotting two shares to the horse and one share to the soldier. That is because a horse ensures more benefit and enables one to launch severe attacks against the enemy better than a man fighting without a horse. This was alluded to in the Noble Qur'an, where Allah, Glorified and Exalted, says: {launching raids at dawn, stirring up thereby [clouds of] dust, and penetrating into the heart of enemy lines} [Al-‘Ādiyāt: 3-5]. The verses refer implicitly to horses and allude to their usefulness in war. Indeed, the Prophet (may Allah's peace and blessings be upon him) said: "There will be good in the forelocks of horses until the Day of Judgment." [Al-Bukhāri and Muslim]