Na verdade, é a Batalha do Portão Negro, para o registro.
De qualquer forma, o wiki da LOTR afirma que, no livro, nós somos de fato cavaleiros a cavalo, cerca de 1000 deles. Eles prenderam os arqueiros de Harad, enquanto os soldados de Gondor lutaram contra os orcs e trolls.
De o wiki ;
Against the Host of the West was arrayed all of Sauron's hordes of orcs, Trolls, and barbarian Mannish allies such as the Easterlings and Southrons (Haradrim). An exact count is not given of the number of Sauron's forces, but it is said that they were "ten times and more than ten times" greater than the Host of the West, making it a force of at least 60,000. Sauron's forces surrounded the Armies of the West on three sides, with the Easterlings on the left flank, the Orcs held the center, and the Haradrim on the right. The Armies of the West always had cohesive stability within their center and kept the Enemy from breaking through by force of numbers. The solid infantry squares of Gondor infantry beat off their inferior Orcish opponents while the Rohirrim cavalrymen staved off the archers of the Harad. The small force of Easterlings launched a quick attack before being repulsed by the armies of Gondor and Rohirrim while the larger Haradrim force slowly retreated to the hills. By now, pressure had eased off of the flanks of the Armies of the West, who smashed into the lines of the Orc-host, slowly beating them off and forcing a retreat back through the Black Gate (though not without suffering sizable losses of their own).
No filme, isso não é explicado porque eles não estão presentes na batalha. A partir da leitura de outros fóruns, a maioria das pessoas diz que Aragorn (e os outros) enviaram os cavalos para um lado, para protegê-los do combate. Lutar a cavalo é uma façanha difícil e, na maior parte do tempo, as forças de cavalaria nas eras medievais só atacariam o inimigo por alguns segundos antes de recuar, para evitar ser puxado de seu cavalo pela infantaria inimiga, fazendo o cavalo cair o cavaleiro no caso de o cavalo ser esfaqueado por uma lança, etc.