I saw the following thread while looking for an animal companion sheet for 2 of my players and i was curious if any one know the issue that is referanced?
http://paizo.com/threads/rzs2jcdf?Animal-Companions
Paizo / Messageboards / Paizo Publishing / Older Products / Pathfinder RPG Prerelease Discussion / General Discussion (Prerelease) / Animal Companions
They appear to be discussing the difference between how D&D 3.5 treated animal companions (they don't grow stronger as you level) and how Pathfinder treats them (they grow stronger as you level). That thread is from either slightly before or slightly after the Pathfinder Core Rulebook was first published.
If you want a plant companion, there's an archetype that lets you get one: treesinger druid. I don't think it would ruin the game to let any druid pick one.