Now that I have a moment to check - there is a bug with the circle. Fixed for v0.13. Sorry about that.
That last druid is definitely tricky to find. Though the secret area she's in has been in the game for a handful of versions now.
Druids are a type of person, typically a sort of religious leader (despite the D&D stereotype that I'm obviously leaning on here). Dryads are a type of spirit (and also Greek, and the mythology I typically look to for MnM is Celtic). As an extra tidbit, though this is mentioned within the game, things like spirits and faeries haven't been seen in the world of the game since before Masters became a thing.
And there are more Estate pets planned. At least one in particular, but I don't know when that'll get implemented.