I have no doubt that horses remember thing for crazy amounts of time (I'm also read that, missnashvilletime!).
I know with Lacey, when I got her, she came to me with her buddy of 15 years: a dark bay Arabian mare. I didn't own that mare but Lacey and Danni (the bay mare) were boarded together. They were never separated except for when one or the other was getting worked, and even then, I'd often ride Danni and pony Lacey since Lacey was still a nutcase and Danni was very level headed.
Then, after having the both of them for about 9 months (3 years ago) Danni tied up really badly and her owner chose to put her down. I'm not sure if Lacey got to see the body or anything, but Lacey was distraught for months afterwards. Once Lacey got over her grief and transferred that bond to me I figured she had forgotten Danni.
But then, a year and a half ago, while we were still at that original barn, someone got a dark bay gelding and Lacey was immediately nuts for him. She would follow him everywhere, eat from his pile of food (thankfully he was a nice guy), etc. I just figured it was becuase he was a gelding and Lacey is a hussy. Well, we moved to camp for the summer and that summer we got a dark bay mare. Lacey immediately ADORED this mare, she basically worshiped the ground this mare walked on. The mare could have cared less about Lacey but Lacey was desperate for her attention.
Then, at the end of the summer, we came "home" to my neighbor's house. It just so happens that one of my neighbor's two llamas (that Lacey lives with) is the llama equivalent of dark bay. You wanna guess which llama Lacey loves to pieces? Yup, the dark bay/brown one. They eat together, Lacey shares everything with this llama. Lacey nickers at the llama and is just ridiculously in love with this llama.
I dunno but I'd say that Lacey is still looking for Danni even though Danni has been dead for over 3 years.
It's really really sad to me but, at the same time, I never need to wonder if Lacey will get along with a dark bay horse. I know that if the horse is dark bay, she'll do her best to be it's bff.
Fabio - 13 year old Arabian/Lipizzaner gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.