My first horse (Arab mare) mainly spooked at things which were no longer there. So - haylage bale in field: fine. Next day, haylage bale gone from field: major spook at space. Cows in field: fine. Next day, cows gone from field: major spook at empty field. It made a sort of sense; I assumed that she was spooking at the fear that whatever was big enough to eat an entire haylage bale or field of cows in a day was obviously big enough to eat her too and might still be lurking.
My last horse, before I had to give up due to health problems, was an ex-racehorse so he was fine hacking in company but took a LOT of work to get him confident hacking alone. His hacking week went something like this:
Monday: Hello trees, hello sky, hello wheelie bin, hello carrier bag, hello cows…EEEK! Purple flowers! RUN!
Tuesday: Hello trees, hello sky, hello carrier bag, hello cows, hello purple flowers…EEK! Wheelie bin! RUN!
Wednesday: Hello trees, hello sky, hello wheelie bin, hello cows, hello purple flowers…EEK! Carrier bag! RUN!
Thursday: Hello trees, hello sky, hello wheelie bin, hello carrier bag, hello cows…EEK! Purple flowers! RUN!
Friday: Hello trees, hello sky, hello wheelie bin, hello carrier bag, hello purple flowers…EEK! Cows! Run!
Saturday: EEK! Trees! RUN! EEK! Sky! RUN! EEK! Wheelie bin! RUN! EEK! Carrier bag! RUN! EEK! Cows! RUN! EEK! Purple flowers! RUN!
Sunday: Hello trees, hello sky, hello wheelie bin, hello carrier bag, hello cows, hello purple flowers. Ooh, are we home already?