So, I have one pasture. I was getting rid of 17+ older horses and transferring others to my barns. However, before I could, I accidentally clicked 'Memorial', and now, I can't undo it. I can't find those horses anywhere.

I think I found a related issue. When in "Card" view the Memorial and the Hay options are still accessible even if the pasture is not empty but not in "List" view. I didn't test it for obvious reason but I would be curious to try to plant hay in a pasture and see if it would lead to the same issue.