A camel is an even-toed ungulate with distinctive fatty deposits, or humps, on its back. It is similar to the real world camel. Camels are used as transport in Essos, including Qarth. A camel cavalry, or camelry, is also used in combat by the Qartheen. 
A Clash of Kings
When Jhogo returns to Daenerys Targaryen and her khalasar in Vaes Tolorro, he brings Pyat Pree, Xaro Xhoan Daxos, and Quaithe. The three strangers are all riding camels, creatures Daenerys does not recognize.
A column of camelry emerge from Qarth as Daenerys's honor guards upon her arrival outside of the city of Qarth. The camelry riders wear scaled copper armor and snouted helms with copper tusks and long black silk plumes. They sit high on saddles inlaid with rubies and garnets. Their camels are dressed in blankets of a hundred different hues.
A Storm of Swords
A Dance with Dragons