In 2018, reddit user u/Metabot submitted a delightful homebrew Background to r/UnearthedArcana: "Was a Goat for 10 Years." With their permission, I've ported the background to Baldur's Gate 3.
The full effect of the mod isn't quite possible yet. The original Homebrew advises having horns and rectangular irises, both of which seem likely candidates for separate mods later on down the line.
A Goat. For 10 Years. Having been a Goat for so long, you know your way around animals, and have become quite athletic, granting you proficiency in Animal Handling and Athletics. You also gain Surefooted, granting you immunity to muddy and difficult terrain. Your irises are rectangular, granting you a small amount of Darkvision, and you've taken a liking to bushy beards.
This mod works with Baldur's Gate 3 Mod Manager, and possibly Vortex and Manual Installations. BG3 Mod Managers is by far the simplest method, and as such is recommended one.