The story
A transliteration of Mongolian Улаанбаатар (Ulaanbaatar), a compound of улаан (ulaan, “red”, adjective) and баатар (baatar, “hero”, cognate to bogatyr). The name was adopted in 1924; before that, the city was known by a number of more prosaic names, including Их Хүрээ (Ix Xüree, “Great Camp”), Даа Хүрээ (Daa Xüree, “Great Camp”) (whence, via Chinese, its English names Kulun and Kuren), and Өргөө (Örgöö, “Residence”) (whence Urga).
Etymology from Wiktionary, CC BY-SA 4.0.