Horses can be "retired" at any age, ie. removed from the game, ie. "dead." Happened to me a few times where someone bought one of my horses just to immediately retire it. Dunno the logic behind it, but it happens sometimes!

People will buy and retire horses for a number of reasons.  I generally don't do it on here unless it's one I bred and re-buy, but I did it pretty extensively on another site.   Some reasons I would personally do it: 

-it's  a loose end from my lines that I have the opportunity to snatch up and remove from the gene pool.  This can make other horses related to it a bit less common.  

- it's just not a great horse.  Everyone forms their opinion on what makes a good horse on here, whether it be stats, color, training boost, etc etc.  A plain horse with a bad training boost/stats.  A 13yo foundation with little to no training/levels. Why have them in the game for others to buy and breed more horses with, when there are plenty of leveled horses breeders wish to sell. -- really, any older foundation with little to no work put into them shouldn't be up for sale and should probably just be retired.  They clutter up the sales pages so much, just pages and pages of unworked, plain, older foundations.  

Again, I haven't done it on here, but I have been sorely tempted to.  If I have the money to buy up an entire string of aging foundations someone is selling from the breeds I'm into to try to clean up the market and the breed - why not?  The seller still gets paid, I get what I want (less clutter), and everyone should go home happy.    On the other side of things I know some people are rather attached to the horses they breed, no matter what they look like or their stats are, and it would be heartbreaking for them.  That's why if I ever intend to do it, I would stick to horses with my bloodline involved or foundations. 

People can often retire horses for something as simple as not being the color they wanted. I see so many good offspring of my horses get immediately retired and I mourn the wasted potential haha but really sometimes it easier to just retire them then waste time trying to sell them

i'm also not really a fan of doing it myself but since retiring is free vs surrendering costing money and reselling taking time, it ends up being the most convenient choice.

it also could be to check BS numbers since you can't see them unless you own the horse. some people just buy up a bunch of horses to figure out the breed's perfect BS and then don't really need the horses afterwards.

and to maybe check what pattern variation a foal has since you can't see that either unless you own the foal.