Evidence 2: Hodor has horse features.
In the novels, after Bran became crippled, he was described as riding the back of Hodor. We also see this numerous times in the TV series. In addition, Hodor seems to be afraid of dark and lightning, very much similar to a horse.
An excerpt from A Storm of Swords goes:
The lightning flashed again, and this time the thunder came at six. “Hodor!” Hodor yelled again. “HODOR! HODOR!” He snatched up his sword, as if to fight the storm.