I would like to know where I could find the rules (both for Pathfinder 1E and the Playtest) to know how much a horse can carry (including a rider).
That's bugging me… Say a horse has +1 STR, which means it would be able to carry a maximum of 11 Bulk (in the Playtest). But how do you calculate how much an Human adventurer would cost in Bulk (along with his equipment)? Surely more than 11 Bulk (if we use the conversion method of 5-10 pounds=1 Bulk found in the Playtest, page 175)!
Or do I just don't care about the horse rider and just calculate the weight of both horse and rider's equipment?
I haven't read the Playtest yet, so can only answer for PF 1E. Carrying Capacity rules are in the Additional Rules chapter of the Core Rulebook. The Bigger and Smaller Creatures subsection states that Large quadrupeds multiply their Strength core's carrying capacity by x3.
For the playtest, the Petrified condition on page 323 indicates that a petrified creature's typical Bulk is 8 for a Small creature or 16 for a Medium creature, and that this is twice their normal Bulk. So normal is ~4 Bulk for Small and ~8 Bulk for Medium.
I believe the playtest lacks rules giving quadrupeds extra carrying capacity, but Large creatures in general get to count 1 Bulk as 1 Light and get double the normal Bulk limits (page 191), so it can carry what a Medium creature would consider 120 Bulk freely and up to a Medium creature's 220 Bulk at max encumbrance.
All in all, I wouldn't worry about the horse being able to carry its rider and their gear... and their loot... and their dead comrades... and maybe a spare horse. That factor of 10 for being Large is heinous (and ridiculous).