When I was 6 months old, our purebred GSD came into our family as a puppy. I was the one who cared for her most of the time. She was my first best friend. When I was 16 (and she was 16) one night we couldn't find her. I was especially worried because she was frightened of thunder and lightning storms and a storm was heading our way. I went outside, looking everywhere for her and eventually found her under a dark space under the side porch. She had tragically lost the use of her hind legs.
I went in the house to get a card table and a blanket. I was going to sit under the table "tent" with her while the storm came so that I could comfort her. When I came out with the blanket she was gone. I couldn't believe it. How could she have gone somewhere when she had no use of her legs?! In the rain I went looking for her. I found her at the front of the house under the spirea bushes.
She was having difficulty breathing. I stayed with her. it was my first experience with death. She may be part of the reason I went into hospice work.
I also have two friends here in Arizona who do not know each other. They both had beloved cats who were pushing 20 years of age. They both told their cats that they did not want to have to make "the decision". Both of those cats seemingly just walked away from home one day and never came back.