I once took a mare on a long horse ride into the Wind River Widerness. We rode the main trail for 7-8 miles and then had to slip off a very small unused trail to reach the lake we were camping out. We made this trip once, In on Monday and out on Friday.
Three years later I returned to that same area. As we rode along the main trail, I was watching for the turn off. This year the whisper of a trail was overgrown, We just had to head off out through a meadow. No visable trail. That mare knew exactly where to turn and head off. I was looking at the topo maps, trying to find my location by looking at ridges and mountains, But that mare knew the way to camp was right there.
I was very impressed that a horse could remember which way to go from a trip 3 years earlier. Especially when it involved leaving the beaten path to travel across an unmarked area.
Last fall, I was up helping a young man with is Moose hunt. I arrived at mid day and the young man wandered into camp early eveing saying he had his moose down 5-6 miles away. We rode off in the early dusk, arriving at the moose at dark. By time we processed the animal and get it packed on the horses, It was DARK. There were no moon or stars that night. We had rode up that trail for the first time ever, a couple of hours earlier. Now we had to find our way back to the truck in the dark. I had one hand holding the reins, and the other holding the lead rope for the pack horses. We set off and I could not see the trail,So I just had to let the horses find their way. We made it back to camp just fine. The horses didn't make one wrong turn.
So both short term and long term I'm impressed with their memory.