REPLY:  Know what worked for me in a similar situation? We had two horses/both gelding quarter horses, one mine (Gabe), one my husband's (Cody), and those two horses had been together as a pair bond for over 20 years!! They weren't just "pair bonded" but were downright "married!!" Well...last fall my horse Gabe suddenly died...he was 26 & had just reached the end of his life naturally. Died peacefully in his sleep, never saw it coming because he was healthy and sound to the end (vet said it was probably a stroke or heart attack, but he'd long been warning me that Gabe was near the end of his days just age-wise). Anyhow, we handled it, was hard, but dealt with it. But Cody was suddenly plunged into SUCH a deep depression. He knew Gabe was dead, I let him see the body (because I didn't want him thinking we sold Gabe away or something and him keeping on hoping Gabe was going to come back). I underestimated how deeply a horse could mourn the loss of a life-time pair bond. And I've never seen such a deep expression of mourning in a horse's face. It's quite a unique look. And heartbreaking to see. And though Gabe had been my longtime partner and I was mourning him myself, I didn't cry unless I looked into Cody's face and saw the deep sadness there -- then it would make me cry.

Anyhow...a week or two went by and though we had another horse (my husband's 6-year-old-then paint, Doc), and they'd all been pastured together, the three of them...Cody still was mourning the loss of his "life partner" Gabe. Since I knew I needed another horse for myself, I got to work shopping around for another. And I came across Sundance who reminded me so much of Gabe it was eery. Long story short, I bought Sundance (after vet check & my training exam). We brought him home and put him in the round pen which overlooks our back pasture.

Well...Cody suddenly snapped out of his depression and ran to meet the "new guy." I introduced them over a fence for a while until I saw that there wasn't going to be a problem, then I let Sundance out with Cody and Doc. For some reason...this was the cure for snapping Cody out of his loss-mourning. He had a new horse to suddenly focus on (Cody is alpha) and "teach the herd rules ropes to" and that kept his mind so busy that he let go of thinking about Gabe. From that point forward. And he moved on. And he pair-bonded with Sundance (though they're pretty much a threesome bond out there) and all was fine. And I realized that was exactly the cure to get Cody back on a healthier emotional track. It worked!

And that's my point -- as it pertains to your situation there. I don't think you're going to see that other horse WANT to pair bond with a new horse right now, as long as the older one is still alive. He's ONLY going to be thinking about that horse, and missing him. From what I experienced here, I think you're better off putting the other horse down first, and...maybe if it works out not too messy/bad putting him down...let your other horse see him dead, because they know "finality" when they see it. And it does let them know: that horse is never coming back (again, that's really a judgement call & depends on circumstances). Here, Gabe died, just putting himself in his stall -- our barn serves as a run-in from several pasture directions -- and he just laid down & died there. Cody saw him in there and I let him enter the stall and sniff all over Gabe's body so that he'd realize: Gabe was dead. Horses know "dead" when they see it, if allowed to sniff all over them. I made that judgement call because I didn't want him thinking we sold off/sent off Gabe somewhere and he'd keep looking for him. And that was the right call there. Didn't stop/lessen the mourning, but a week or two later adding a brand new horse to the mix DID halt his mourning and gave his brain something new to focus on. And it worked.

I think something similar to that would work for you, too. But I have to say...had I tried to introduce a new horse in the mix while Gabe was around, like...if I was planning to put Gabe down, like you are called upon to do there (and it's the right decision, know that), I just don't think Cody would have given the new horse the time of day or that it would have helped lessen his mourning. Because all he cared about at that point (when Gabe was alive) was keeping his pair bond with Gabe--and pushing any other new horse away so they didn't steal Gabe from him. But it did work AFTERWARDS. And it helped him to move on faster, getting over Gabe's death. He had to think about the new guy 24/7 and deal with him, and that's about all his brain could wrap around. By the time they settled in, the mourning period was over. Get it? So...that route worked really well for us here.

And that's why I wanted to share that story with you, because I have a feeling it would work for you there too. Meaning: don't worry about trying to introduce the/a new guy just yet. Put the older horse down, let the pair-bonded one know that the horse is really gone (if that's at all possible), THEN introduce the new guy a few days later maybe. He'll accept it more easily then because he won't want to be without a "friend." You might try that and I think it'll work. I was just feeling my way through our experiences here, but the route I sensed would work, did, in the end. So...just wanted to pass that along for whatever it's worth to you. Hope it helps!