Related: Bring a horse in a dungeon. How is this question different? I am asking specifically as a mounted archer, who doesn't need to charge on horseback and does not have the option of choosing a different mount.
In the current Pathfinder campaign I am a human ranger and have a combat-trained light horse animal companion.
I have placed a number of points into the Ride skill (and also Handle Animal) and am appropriately outfitted to annihilate my foes from the saddle as an archer (composite longbow). However, most of the combat encounters take place in dungeons where my large-sized horse is, well, uncomfortable to say the least.
Now, this is not a matter of squeezing, but of how the horse can actually still be useful in small spaces.
In this case I am not looking to replace my mount for RP reasons, so they would need to be solutions that work with a large-sized horse (unless shrinking is involved). As I am also exclusively an archer, I would still need to be able to keep some distance between myself and my target when possible (ie. when space permits).
Question: What are some ways (if there are any) I could continue to ride my horse effectively in combat while in a dungeon?
Best Answer
Why not run the following ideas past your DM
Combat
1) Some sort of horseshoes, saddle, etc. that would allow the horse to fly/ levitate / swim in some manner. This gets around the limitation of the horse needing to be on stable ground.
Included in this idea is polymorphing your horse into a Pegasus (fly) / Hippocampus (swim) or other such animal.
Limitations: the horse would need to be trained to fly (swim) as a trick - which may take awhile. You as the rider might have to take the fly skill, since you're flying your horse.
2) Conversely, get it a ring or feather fall and boots of spider climbing (well in this case it would be "horseshoes")
Limitations: The horse might not want to use the magic items (because it's scared of heights, because even though it knows it's going to be safe, it's tough to overcome natural instincts.)
3) Platforms (think several Tenser's Floating Disks") as a stable, immovable platform. (For ideas on how to use if you need to have it move, see this link on giant tip Get a wand or several scrolls of them of them to save from having to get levels of wizard.
Limitations: The wand/ scrolls caster providing caster level.
4) Teach horse tricks to deal with a dungeon area, if you've not already done so.
Limitations: Keep in mind that even with tricks, the horse is going to have to typically go slower than it normally would because of stalagmites/ stalactites, slanted, unstable, slippery, narrow, areas, and would automatically count as difficult terrain for the horse, unless the DM specifically states the area as having no hazards that the horse would be able to get injured on or by.
Getting Horse from Point A to Point B
(included for completeness Sake)
5) Reduce Animal + bag of holding (provided the duration is 10 minutes or less)
6) Rope Trick
7) Tower Shield with Hosteling
8) Carry Companion Spell
Ideas taken from here: this reddit thread