Green’s Dictionary of Slang

(US) an outstanding exemplar, i.e. the greatest of all time.

2004		[bk title] Goat: A Tribute to Muhammad Ali.
2016		C. Eble UNC-CH Campus Sl. Spring 2016 5: GOAT — greatest of all times someone or something fun, appealing, praiseworthy: “My mom made my favorite meal when I went home this weekend. She is the goat’.
2021		Twitter 12 Dec. 🌐 Lewis [Hamilton] the true champ and the goat.