https://en.wikipedia.org/wiki/List_of_equine_fatalities_in_the_Grand_National

(If multiple horses die that counts, it just has to be one or more.)

Edit: If you like this market, you may be interested in my other market, asking if perhaps more than one horse will die at the Grand National in 2024.

https://manifold.markets/asmith/will-two-or-more-horses-die-at-the

Edit: I didn't realize that there are two ways to interpret the market title! Two horses died at the Grand National event weekend, but none in the Grand National race itself. It's (kind of) like whether non-combat deaths should be counted as casualties in war. I am sorry for this ambiguity in the question. I feel that the most fair way to resolve this is to resolve this N/A. I apologize for the lack of foresight; I didn't foresee the possibility of ambiguity. I also apologize to those who feel that this is the wrong way to handle the ambiguity, sincerely.

Edit: "Negative payouts too large for resolution. Contact admin." Ouch! I'm going to resolve 50%.