Jingles for your horse. Very scary and I’m so sorry.
Years ago my vet gave my Connemara the then “new” equine specific rabies vax. The mare was always fine with vax.
About 3 hours later I found her laying flat out and unwilling/unable to move. Doc returned. I don’t recall any of her vitals but she responded positively to banamine and whatever else Doc gave her.
The company paid for the emergency call in full and I believe the vax was pulled off the market within a year.