What an inane, blithering crock! So they “evolved” to where they could srvive the cold over 800 years, did they? And how, may I ask, did the original arrivals from Mongolia survive the cold for the first 500 years or so.
Answer: God designed the original equine progenitors with the genetic potential to withstand cold when it is encountered; as well as withstanding heat, as Arabian horses do.
These fraudulent faux scientists should be laughed out of intelligent circles.
Just how warm do you think Mongolia is?
They didn't. You would have to get horses that survived somewhat less harsh conditions further south, and keep importing horses north until you got some that were able to survive. Then repeat the process to get further north.