White posted the news on Facebook on Sunday, just hours after he reported for training camp and signed his rookie contract. Daisy Mae was set to be shipped from Cotton Valley, Louisiana, to a stable in Tampa on Sunday to be closer to White.
The horse was not sick but pregnant, and collapsed Saturday night because of heat exhaustion and dehydration, a source told ESPN.
White, who referred to Daisy Mae as "my daughter" and "first love," celebrated the horse' birthday with her May 31. He had gone back to Louisiana to be with her a little over a week ago.
White kept the horse at a stable near the LSU campus and rode her out onto the field at Tiger Stadium last November, calling it "the best moment of my life."