I inadvertently killed the dog today.
I took the little one over to her friend's house and the dogs went with us. They couldn't be left at home today. The little one's friend used to live next door, but they moved to another house close by a few years ago so they would have the space to keep horses. They have 4 horses, a cat, and a dog.
The boxer was thrilled to have a dog his size to play with, and he was excited by the horses, not to mention the normal excitement of being around people he doesn't usually see. I kept the dogs on leashes the whole time as we did not want them messing with the horses and getting kicked or stepped on. That meant that the dogs didn't really run while we were there. Still, the boxer kept pulling at the leash wanting to go here, and here, and here, and ooo look at that! He played off and on with the dog of his size, the two of them pawing at each other and doing play-biting, etc.
Unfortunately, the boxer had heart problems. He had had a couple of seizures in the past. When he had his seizures it came when he was running around a lot, as puppies (and especially boxer puppies) will do. The boxer wasn't running around, but I didn't really consider what 2-3 hours of the excitement of being at a place with people he didn't know, a dog he didn't know and could play rough with, and horses (which he'd never seen before) would do. After some play with the other dog, the boxer lay down and put his head on his paws. Our former neighbor commented that she thought he was going to sleep. After a minute he started to have labored breathing, then he lay on his side and his tongue came out, and his breath got more labored. After a short time his breathing stopped and did not start again.
We put the two littles in the house to play before the dog died. We kept them playing while my employer drove out to the house. She told the little one, who of course was bawling. We've been home for an hour+ and the little one is still bursting into tears now and then.
I know this sort of thing was probably inevitable, given the boxer's heart problems, but I should have considered how the excitement might affect him and kept the day short. Who knows how long he might have lived had I not exposed him to all that excitement?