I had this happen when I fast traveled back to an Act 1 area while the Halsin quest was active. I got his backpack in my inventory after fast traveling and then he disappeared completely from my camp and Last Light. Had to reload a save to get him back.
The great thing about BG3 is because the game hews so close to the 5e rules, a lot of build guides for tabletop are perfectly valid:
https://rpgbot.net/dnd5/characters/classes/#druid
Do note that the way Wild Shape works is one of the biggest differences in BG3 from 5e (can’t use abilities from multiclassing like Rage or Flurry of Blows in wild shape, auto-scaling animal forms, etc.), so take it with a grain of salt.