Howdy,
Bushhog 2615 w/rings behind a JD 6400 90-95hp. Works nicely thru thick and thin. Hills too.
I have also pulled it behind my JD 5500 75hp, with rack and pinion with wheel weights and fluid filled tires. The hp is there, but gets a little touchy around hills and turning. :)
The others are correct. I would suggest 70 hp or higher for a 15' batwing.