There's also Tregellus Aranega, a neutral male human Druid of the Green Faith 15. He's the Grand Druid (given that Paths of Prestige has a Green Faith class, I'd probably re-stat him as a Druid 5 / Green Faith Acolyte 10 to give him the Grand Druid capstone, but either way it's still 15 levels). Going by his description and age (born the year before Aroden's death), I'd say he's also venerable and very close to the end of his life.
There don't really seem to be any level 15+ druids, at least in the region of the Inner Sea. I wonder why that is.
I also went and checked Mythic Realms. From that, I got the following:
- Mordant Censor patrols around the Spire all include a level 13 druid amongst their number
- Lenas (LN male pod-spawned human Druid 10 / Hierophant 5) in the village of Haven, in the Tusk Mountains
- Old Executioner of the Mountains (NE variant mythic treant Druid 10) is a mumbalam treant in the Sudisan Peaks of the Valashmai jungle
So there are a few others. Not many, though. The 15th level ones are pretty much as high as you can get, unless Old Executioner counts (the mythic treant aspect likely kicks its CR up to around ~20).
EDIT: I spoke too soon. Prowling through the Worldwound book reveals one more.
Auzmezar, Master of the Circle of Hierophants (NE male human siabrae Druid 20 / Hierophant 5). So that would make him a CR 23 opponent, possibly CR 24 if he has PC wealth. He's the highest-level druid on Golarion, and possesses a solid helping of mythic power to boot. He's also an undead siabrae abomination, of course, so that could make him an excellent end boss for a druid-focused campaign (particularly given the Worldwound and Auzmezar's connection to the now-corrupted Green Faith stuff there).
EDIT 2: And yet one more mid-level one from Dragons Unleashed.
Skaldreg (NE male centaur Druid 12). He's not on par with any of the 15th level druids, or Auzmezar, but he leads a clan of centaurs in the service of Sjohvor, a Great Wyrm White Dragon in Iobaria.
