Which Alliance race was introduced in The Burning Crusade?

The Alliance race introduced in The Burning Crusade was the draenei.

The draenei joined the Alliance alongside the blood elves, who joined the Horde. In the expansion’s story, the draenei are portrayed as uncorrupted eredar led by the prophet Velen. Their arrival connects the Alliance to Outland and to the naaru, ancient beings who helped oppose the Burning Legion.

The draenei began in Azuremyst Isle, off the coast of Kalimdor, and their capital city was the Exodar. Their addition also changed class availability: draenei could become shamans, giving the Alliance access to that class, while blood elf paladins gave the Horde access to paladins. Gnomes and night elves were original playable Alliance races; worgen arrived much later in Cataclysm.