Earliest abolitionists

Question is inspired by Game of Thrones, some mouseover space for a mild spoiler;

In the show, set in a medieval fantasy world, one of the characters Daenerys Targaryen is a staunch abolitionist in a violent John Brown kinda way and grants manumission to any slaves she comes across. Got me thinking - how much of an anachronism would this have been in reality? Someone who thought the institution of slavery was an evil that should be destroyed in a world full of it?

Obviously the 19th century is when abolitionism got its legs - were there significant figures for the movement long before that, even in antiquity?

The earliest I can find, hopefully you can help me find some more, is Gregory of Nyssa, a bishop who lived in the fourth century and condemned slavery in religious terms (much as the more famous abolitionists like Wilberforce and indeed Brown would centuries later).

Are we limited to Americans, and the “modern” institution of slavery? If not, I’d toss in Cyrus the Great of the Achaemanids and Wang Mang of the Chinese, both of whom abolished it.

By no means, these were the kind of examples I was looking for. Particularly Wang Mang who I wasn’t familiar with, looks like he was forced to repeal the abolition of slavery which must have been a bit…awkward.