
I may have mentioned this before, I strongly believe that the Mithras known to the Romans was no relation to Zoroastrian Mitra, but in actuality a culturally-appropriate adaptation of Horus himself. This is something I've researched in depth for my upcoming book on esoteric symbolism in science fiction film.
Suffice it to say that conservative Romans held a very dim view of Egypt, seeing it as corrupt and decadent. And they also thought the notion of animal heads on gods was offensive. But the Shemsu Hor obviously had a very strong interest in Rome, and it's my belief that the Mithras cult was created especially for those Romans whom they wanted to bring into the fold, namely those alpha males in business and the military. The Shemsu Hor's fingerprints are all over Mithraism, as we will look at in the future.
All of these figures - Horus, Mithras, Helios, Sol, Shamash, Ba'al, Hercules, etc etc etc - all represent the Sun as a symbol of phallic aggression, technology, hierarchy, and militarism, so any difference between them or their cults is academic, anyway.