My first horse had two hard months in the spring. He was always a reserved horse who wouldn’t admit he was missing a leg if the tiger was still attached, but I could tell he wasn’t quite himself, and he wore his toes too fast (arthritis in his forelegs). When summer came he perked up and was his normal self. But I could predict more hard months - fall, a warm spell in the winter. They could easily add up to five months a year - which I felt was too much time to be enduring life. So I gave him the summer and euthanized before we got to wet, cold, damp fall weather.
Second horse lost his worry. He was always a chicken, and worried about everything. The more pain he was in, the spookier he got. That last summer he was at peace. He’d lost a great deal of strength in his hindquarters, started moving away whenever one of the younger horses acted up in his direction (never used to take guff from any youngster before), and even stood differently when snoozing in the field. Nothing anyone else would notice, just me because I knew him. He’d dropped some weight and never got it back, but superficially he looked good and many people commented on it. He did still spook at some things on the trail, not that we went out much because it was painful for me to feel how weak his hindquarters were. I discovered the right question by accident after I’d decided to euth before winter - we all ask if we can keep the horse going and the answer is almost always yes, but I asked should I keep him going? Was it right to prolong his life just because I could, and because I wasn’t ready to let go? I had a fear of him going down on the ice one winter night and being unable to get back up - and I just couldn’t do that to him. His being at peace, losing the worry, moving away from any conflict all seemed to be his way of saying “I’m done fighting” and I had to respect that.
My first horse taught me a bunch of things about knowing when it’s time and I posted that knowledge on various BBs over the years, distilling it into a set of guidelines that I used when it came time to decide about my second horse. That info is here - it might help
http://endgame-journeys-end.blogspot.com/
You will know. But only if you’re paying attention. Easier said than done when it’s our beloved friend.