Here's some info of the testing I did with Erebrandir's Horseshoe in Instance: An Echo of Days Past:
I went through Instance: An Echo of Days Past many times both wearing the horseshoe and not wearing it.
- The first "ghost" appeared every single time, no matter if I had the horseshoe equipped or not.
- The second "ghost" always appeared only when wearing the horseshoe.
- I also went to the second "ghost-sighting" spot many times while not wearing the horseshoe and seeing no ghost, and after a while I equipped the horseshoe, and the ghost appeared the second I equipped it.