Thanks for the question.  I'm going to let the mythology experts deal with the specifics, but I want to caution you against falling into a trap.  Many Christians, when they defend the Jesus myth as unique, will look at Horus and Mythras, et al, and retort with, "Well, yeah, they were saviors, but not of all mankind in exactly the way that Jesus was."

In other words, they will discount any previous myth that isn't just like the Jesus myth.  This is obviously a bad argument, for if there was such a myth, it would be Jesus, wouldn't it?  They would then use the existence of such a myth to prove god -- calling it foreshadowing, or maybe even saying that it was the true Jesus.  Whatever.  The point is, a myth doesn't have to be exactly like Jesus to be a savior myth.

In Greek mythology, humans used to be two sided, with faces, arms, legs, and genitalia, in pairs.  They were immensely happy, and were capable of incredible acts of sexual prowess.  They became proud and haughty, and Zeus was angry with them for their sin of believing themselves to be godlike.  As punishment, he split people down the middle, exposing their spines, and severing them, so that they were now imperfect.  They were clumsy and could no longer maintain their balance.  They couldn't see behind themselves.  Worst, they could not experience the kind of sexual joy they once knew.  

After mankind had suffered long enough, Apollo took on the task of giving them relief from their punishment.  He took their skin and stretched it around them, covering their spines.  He tied it in a knot at their navel.  He moved their genitalia to the familiar spot.  Though humanity was not perfect, they now looked better.  They were no longer hideous.  Finally, Apollo gave them the ability to find their other half.  Before the punishment, there had been three types of people -- men, women, and men/women.  Some had both male halves, some both female, and others were half man and half woman.  After Apollo's reconstruction, man had an innate desire to find his other half.  Sometimes, he longed for a man.  Sometimes, a woman.  And sometimes women longed to find their lost female half.  Though most people never find their true mate, some do, and they are happy for their entire lives.

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It's not even remotely like the Jesus myth, but it is a myth about a savior.  Instead of saving us from eternal damnation, Apollo saved us from intense earthly loneliness and sexual unfulfillment.  In many ways, I'd say the Greek myth is far better than the Christian myth, for it gives meaning to our lives, and acceptance to our sexuality.  It gives us purpose in this life, not just the desire to get it over with so we can go to heaven, where none of this matters anyway.

The point is this:  There are many savior myths, all over the world, and many of them predate Jesus.  To say that there's only one kind of savior myth -- saving people from eternal damnation -- is incredibly myopic, and damn ethnocentric.

 I didn't get causation from the question.  I thought he was just asking if Jesus was the first genuine savior myth.  If that's the case, then clearly the answer is hundreds of previous myths existed.  I was trying to think of one that was particularly far away from the Jesus myth literally as well as figuratively to demonstrate just how mundane the idea of a savior myth was.

While I don't think it's possible to draw a straight line correlation between a particular Greek myth and Pauline Christianity, I think it's beyond debate that the pre-Christian Jews were culturally influenced by the Greek and Persian cultures, namely in their inheritance of the Male dominated negative theology of human nature, and I think, by their concept that humanity needed to be saved from something.

Whether the exact nature of the salvation can be considered a descendant of a specific myth, I would think the degree of cultural interaction was sufficient to establish a chain of continuity, at least in the overall conception of the cosmos and the corrupt nature of man.

Agreed.  Yours is later than mine, if I remember correctly, and would have been much more familiar to Jews.  As I said, I was going for more ancient.

A great question.  I'd love to hear your theories.

Indeed, I think the very question of correlation and causation between Jesus and previous savior myths betrays a hidden agenda (Not necessarily in the OP's question -- but by theists in general) and is not productive to answering questions as it is in furthering the so-called debate between mythicists and apologists.

There was a podcast of Bob Price (possibly a Bible Geek episode) in which he mentioned a guy roughly at the time of Jesus who was declared a God, claimed to be a God and did miracles. I think his name started with an "s" and about 5-6 letters long (if memory serves).

Does anyone know what I'm talking about?  Often Xian's will make the case that a myth can't grow out of a true story so quickly - this contradicts it but its hard to find and source.