Discussion: How much do you think a pony weighs? · 8:39pm Jan 27th, 2013
Their height can and already has been established to 4ft, Celestia being at least 6. I would consider a foal to be at least 2ft.
I would consider a pony based on their stature to be about... 180 to 350 pounds, of course varying with wings/ gender/ height whatever.
But that's just guesstimating.