The Torah commands in Parashat Shoftim (Debarim 17:16), “Lo Yarbeh Lo Susim” – that a Jewish king may not acquire extra horses. The Rabbis explain that the king may acquire as many horses as are needed for the military, and, of course, he may have a horse on which to ride. However, he may not acquire even a single additional horse for himself. Although the Torah here forbids acquiring extra “Susim” – “horses,” in the plural form – the Torah later in this verse makes reference to just a single horse (“Lema’an Harbot Sus”), indicating that acquiring even just one unnecessary horse is forbidden.
As the Sefer Ha’hinuch notes, the Torah itself provides the reason for this prohibition, explaining “Ve’lo Yashib Et Ha’am Misrayema” – that acquiring excess horses might lead to a return to Egypt. The Sefer Ha’hinuch explains that if the king begins expanding his cavalry, he might send servants to permanently reside in Egypt, where quality horses were available, in order to purchase and breed horses. The Torah forbids permanently residing in Egypt (as will be discussed in Misva #500), and so the Torah commanded that a king should not acquire extra horses, to prevent against permanent residence in Egypt.
Additionally, the Sefer Ha’hinuch writes, acquiring excess horses – which have an impressive appearance and gallop quickly and gracefully – could lead the king to arrogance and conceit. For this reason, too, the Torah forbids kings from acquiring unneeded horses.
This command applies only in Eretz Yisrael, and, of course, only to the Jewish king. A king who acquires even a single horse unnecessarily transgresses this prohibition, and is liable to Malkut.