What do personality traits change?
How traits are discovered
The pictured in-game Library explains that interacting with horses reveals traits over time or in relevant situations. Breeding-related characteristics may become apparent when a foal is bred. It does not give a fixed number of interactions or a universal discovery timer.
Testable is different from inheritable
The developer screenshot displays separate flags. Easy Keeper and Hard Keeper show Yes for both. Snacky and Cuddly show No for both. A confidence-related quirk is therefore not automatically an inherited breeding target. These flags describe the pictured pre-release build, not a newly tested release build.
Use the partial catalog
Compare the names and effects below with the wording in your current game. Add the exact wording to your herd notebook, retain your version, and keep unknown values blank. Neat Freak and Good Inheritance are names only in the available Horse Info image; their detailed mechanics remain UNKNOWN.