I was planning on making a sorcerer with a giant dragonfly companion (yes, sylvan bloodline) and I've more or less figured out how to deal with the intelligence issue. What I was surprised to find though was the "curious companion" feat which allows you to choose a plant or vermin companion and well, I thought those were options allowed by default?
The feat seems to be from wilderness origins and I could've sworn that vermin and especially plant companions were around before that, so I'm a bit confused about if I'll need to grab that feat too.
Not a pathfinder society campaign by the way.
by the RAW druids can only pick an animal
The second option is to form a close bond with an animal companion. A druid may begin play with any of the animals listed in Animal Choices. This animal is a loyal companion that accompanies the druid on her adventures.
plants aren't animals, they are plants. You are correct in thinking that plants have been around as companion choices for longer then wilderness origns. However, you needed to have the ability to pick a plant companion. For example a treesinger can have a plant companion.
To be fair, it is confusing because by RAW vermin aren't animals either, but they are called out as being viable choices for druids.
In addition to the normal choices of animal companions, a druid who is so inclined may select a vermin as her companion.
The plant companion entry lacks this phrasing.
I will point out that, some classes and archetypes limit what kind of animal companions you are allowed to have. For the cost of 1 feat curious companion opens things up dramatically for those characters.
In your case since it just references the druid animal companion ability without limitation, if you don't have the curious companion feat you can have a vermin but not a plant companion.
It's normally an ask-your-GM thing (unless you're the right archetype as LordKailas said).
Plant companions are typically suitable only for specific archetypes, such as the treesinger druid archetype (Pathfinder RPG Advanced Class Guide 25), but with the GM’s permission, any druid could select a plant companion.
Of course that does say "any druid," so Curious Companion is still needed for other companion classes.