Pictogram (象形) – originally a serpent with prominent whiskered mouth and eyes.
Current form developed in large seal script, with serpent’s body on right (tail at upper right, legs on right), whiskered/fanged mouth at lower left, and eyes/crown at upper left. Left side was subsequently simplified and abstracted, with some influence of 立 and ⺼/月. Note that 竜 existed as a traditional variant dating back to large seal script, and figures a dragon seen face-on, rather than curled around.
From Proto-Sino-Tibetan*m-bru(ŋ/k) (“dragon; thunder”). Cognate with Tibetanའབྲུག ('brug, “dragon; thunder”). The STEDT database also lists 隆 (OC *ɡ·ruːŋ, “thunder; sound of thunder”) and 雹 (OC *bruːɡ, “hail”) as cognates. Also compare 靐 (OC *brɯŋs, “sound of thunder”) and 霹靂 (“thunder”).