Why not just give your horse a hoof supplement instead?
Even IF Jello had any hoof benefits, and that is a big IF, the amount you would need to feed a horse to see any benefit would be by the bucketfull...and your horse would probably colic or get laminitis from all the sugar. JMO.
Doesn't seem safe.
What I WOULD reccomend is Platform Hoof supplement. The Belgian draft in the photo has been on it for a year. Her hooves have NEVER had a crack or chip, NOT ONE...not even during fly stomping season...AND she is overweight. I swear by this stuff.