As an act of curiousity, I'm trying to come up with a list of all bossfights in Paizo APs where it would be reasonable to have the fight while mounted.
Let me do some definitions:
"bossfight" is loosely defined to mean:
- a battle (usually against a solo opponent) that has a CR significantly above the party's level
- the fight takes place at a transition-point in the story (a boss cannot merely be in the middle of a dungeon)
- note: a boss does not have to the the BBEG at the end of the book
"while mounted" I'm a bit stricter with:
- it was possible and reasonable to get your mount to to the boss without having to resort to summoning or other tricks (unless the PCs themselves were forced/assumed to use the same trick)
- "a mount" is assumed to be a large creature with limited dexterity (a horse cannot climb a ladder, for example)
- that there is enough space to run around, say to use the Ride By Attack feat most rounds
- the boss is either mostly land-banded, or the PCs are sufficiently high level to assume a flying mount (I'll roughly peg this at level 9, since that's when Wizards get Overland Flight)
RotRL:
- Jarl, from near the end of Hook Mountain Massacre, was at the end of an oversized cave-complex, and his fight was in a large courtyard
- the White Dragon, outside of the Runeforge, was perfect for a mounted battle
KM:
- the Owlbear at the end of Rivers Run Red is in an oversized cave complex
- the Jabberwock that attacks your capital city in Sound of 1000 Screams is perfect for a mounted battle
- the Ancient Black Dragon in the Forest of 1000 Breaths is also great for a mounted battle
SS:
- I don't know if the Froghemoth in Saventh-Yhi is a boss (I don't think it meets the criteria), but it's definately doable mounted
CC:
- You could make the arguement that the werewolves atop the Stairs of the Moon were mount-accessable, but I don't think there's really enough space to move around up there
- The Undead Dragon in the final city of Adorak would make for a good mounted combat, once the characters can get a bead on him
I can't think of any others myself, but I haven't read every AP.