Then began the mariners to open the ports of the transports, and let down the bridges, and take out the horses; and the knights began to mount, and they began to marshal the divisions of the host in due order.
Geoffrey de Villehardouin [b.c.1160-d.c.1213]: Memoirs or Chronicle of The Fourth Crusade and The Conquest of Constantinople, trans. Frank T. Marzials, (London: J.M. Dent, 1908)
So the fleet came to land, and when they were landed, forth came the knights out of the transports, all mounted; for the transports were built in such fashion that they had doors, which were easily opened, and a bridge was thrust out whereby the knights could come forth to land all mounted.
Robert of Clari's account of the Fourth Crusade
Those sources called the horse transports uissiers. Other names included chelandium, tarida and dromon. They were big galleys capable of carrying 12-30 horses. The big thirty horse taride of Charles I of Sicily shipped 108-110 oars. The doors and ramps were at the stern between two sternposts, so the vessels backed onto the beach to unload and load. They were shallow draft: in Villehardouin's account the knights jumped from the transports into waist-deep water.
The vessels are covered in more detail in: Bull, M. (2003). The experience of crusading. Cambridge: Cambridge University Press.
More on the ships and the landing here.