I priced out materials to build our sheds. They were all 12x36.
6 6x6 posts in the ground. Framed out w/ 2x6’s. 5’ kickwall made of 2x12x12s. Tin shell and roof. Materials were $1400 and labor for someone else to install was $1000 (friends construction crew).
You may be able to build a permanent one that you won’t have to worry about colts eating apart (!) for around $700 if you can build yourself. 2 colts surely wouldn’t need a 12x36. Maybe a 12x20 if they are realllly friendly!
If you know any contractor friends- I’m sure they would be willing to help out squaring the building and such for pizza and beer!
Good luck!