Neighbors Horse Died

Went out to check my two horses early one morning a few weeks ago and I noticed one of my neighbors horses was down. I walked over to check her, but she wouldn't respond. So I went inside to call me neighbor and make sure they knew one of their horses was down. They said they knew, that they found her the night before. It is not known what killed her, but all the other horses are bright eyed and happy, so life goes on... Still freaked me out a little, thinking that could have been one of mine. Here is the lovely photo...