If they like to chew those sharp corners won't last long anyway. I have heard they don't like the taste of hemlock.
More important would be to cut all pegs nearly flush even those up high.
Is this a walk in shelter? Open sides or maybe 3 sides closed for wind protection?
I almost built a simple H-bent frame, 2 bents, 16'x16', top plates and common rafters, like the Sobon shed but with out all the girts. Very simple and straight to the point. You would even have a little space above for who knows what, why waste it. Just lay some slabbed 2 sides poles on top of the ties for joist, no layout or joints. But I never got around tuit, and the old horse is gone to a new home with more horses.
A 10'x10' box stall is typical, would depend on the size of the horse.