Horus is a pre-dynastic God. His history reaches far back to Egypts origins.
Menes (also known as Aha or Scorpion) 3100-2850 B.C. had Horus in part of his names. His 'Horus name' or his devine name. (the Pharaoh had as many as five names, adding to our later confusion.)
By this point Horus was a fully formed god, with many temples and a well stuctured priesthood. So it is likely that Horus' origen goes back eaven further although writing is still in its formitive stage making it hard to find 'written evidence'.
I think calling Christianity a mix of other religons plays down the achievement that christianity made. In finding ways to absorb other religions quickly and seamlessly it enabled it to spread as far and as quickly as it did. All religions borrow from one another in varying degrees, but Christianity took it to a whole new level enableing it to 'swallow up' other cultures and thier religions. An idea, incedentaly, they may have borrowed from imperial Rome.
Christianity today, in the world of global communication, does not do this anymore. The phenomina was IMHO restricted to the early history of the religion when communication was more difficult and thus such tactics were easier to get away with without raising the ire of the rest of the faith.
The last time I think this happined was when Christianity began to incorperate aspects of the Mayan and Aztec and Incan faiths into itself to spead to the natives of Central and South America.
Or perhaps it is the clever action of the older 'pagan' faiths to keep themselves viable and relively free from persicution?
"Hey, you what are you doing? Praying to some banned "evil pagan" deity?"
"no, no we're not praying to Brigit(Brigantia) the Goddess... we're praying to Brigid the Saint."
"oh, carry on then, carry on."
I'm just waiting for St. Coyote...
wa:do