Back in day, a bunch of people—you know, ancient writers—used to say that the Greek Io was the same goddess as the Egyptian Isis. Why? Well, both were often shown as beautiful young women with the horns of a cow. (How long till this look shows up on the runway in Fashion Week? Actually, it probably already did and we missed it.)
On the other hand, there are folks that say that the Greeks made up all this stuff and that Io and Isis don't really have a whole lot do with each other. Or at least that the Greek's got Io's look from Isis and not the other way around. Word on the street is that Isis actually got her headgear from a super ancient cow goddess named Hathor, who chilled in the sky with the golden disk of the sun blazing between her horns. (Must've gotten hot.) As Isis got more and more popular, she took over Hathor's duties and her headgear.
So, who copied whom? Did Io turn into Isis? Did Io just copy Isis' style? Or did they all just take a page from Hathor's fashion book? It's hard to know any of this for sure, since this stuff happened a seriously long time ago, but there's no doubt these goddesses all kicked the same cow-horned style. (OMG, it would be so awkward if they showed up at the same party.)