Hmmm, my hobby horse was a single plastic molding mounted to a frame with springs. Not what you're doing, obviously.

If I were using fabric or fiber to make these parts, I'd be concerned that

1) The mane be strong enough for the kid to hold onto safely while rocking back and forth,

2) Both parts, but especially the mane, be dirt-resistant and/or easy to remove and clean.

Would real horse tail work? If so, I may be able to help.

Nancy

Mop head. Strong enough, can be cleaned without fear of any problems, and can be dyed.

Sisal....a rope un twisted, wed and hung to dry.
Or crepe hair from a costume shop..It won't be strong like hemp or sisal but will be cheap and easily replaced.
Linda C