I would suggest finding a tack store somewhere and buying an actual horse riding helmet.
I bought mine for $60 and I don't feel it looks terrible, it's very comfortable use it every time i'm on a horse.
bike helmet is to take one blow, an equestrian helmet is designed to absorb multiple blows where if you are hung up on the side of a horse and hitting your head off the ground a few times before you can get yourself freed.
it's one thing i've thought about and if I go anywhere and ride an unfamiliar horse, I would be that much more inclined to wear a helmet
