Zeus was doing his usual Zeus thing, chatting up a mortal priestess (ancient greek devouts were so much open to seduction) named Io, when Hera came along and was all like "Zeus, you better not be hitting on some human again?!" and so Zeus, being that he was the king of the gods, was crafty and quick enough to change Io into a cow before his wife could catch them together.
Hera though, being that she was the Queen of the gods and also, you know, knowing what Zeus was like, was suspicious when she found her husband with his arms wrapped around a cow and so, using her wit, proclaimed "Oh, what a fabulous gift you've got for me there, I've always wanted a cow!" and Zeus was forced to give over poor Io to Hera.
However, thinking quickly, Zeus burst out "Ah, but we live on Mount Olympus, can't take cows up a mountain, they're very terrestrial, scared of heights, don't you know."
Hera conceded the point but insisted that the cow be kept in her sacred olive grove (somehow not a euphemism) and watched over by her friend, Argos, who was a giant with like, so many eyes, just, freaking, so many, it was weird. Hera knew that loyal Argos would keep an eye (or two, or fifty, or a hundred) on Io at all times because he never closed them all at once.
However, she had not banked on Hermes, who Zeus sent to kill Argos because that's the logical reaction to being cockblocked by your wife. Send your son to kill her friend. Anyway, Hermes went there and tried to get Argos to leave the cow unattended, but no matter what he did he couldn't get the giant to close all his eyes at the same time. He tried spells and hexes, he tried playing soothing lullabies on his lyre, nothing would work until he started spouting on about the history of musical instruments ("Hey, did I ever tell you how I made the first lyre? First, I got this turtle right...." and on he goes, oy) which knocked the giant out. Then, Hermes lopped Argos' head off with a sickle because fucker was hardcore.
As it turns out, Zeus had kind of moved on by this point and didn't really care anymore. He went off to do Zeus things. Argos' eyes got put on peacock tails because they were Hera's favourite bird. Hermes got no comeuppance. And as for Io, well, she eventually got turned back into a human, after being cursed by Hera to madness by gadfly (just one, that couldn't be gotten rid of, just stinging her, all the time, to induce insanity you see), meeting the titan Prometheus and swimming across a whole fucking ocean (as a human that's tough, as a cow...yeah). But it was all worth it because in the end Zeus had his way and she ended up giving birth to some of Zeus' kids (sure, he had given up on her, but Zeus is Zeus), who then had kids of their own, who then had kids with other Zeus babies because looking at the family tree of ancient greeks myths is a lot like reading a rednecks family tree (oh my mother is my brother, is my fathers aunties uncle, is my grandpa, is my nephew is my son! and so on, seriously, zeus hit up his own grandchildren multiple times, who then procreated with each other, and don't even get me started on Herakles and the Danaids, 1 Zeus male descendant, 50 Zeus female descendants, and all 50 were impregnated, and then started marrying each other, and all the ancient greek rulers pointed at shit like this and was like "we is descended from this union, this is why we're more specialler than you!" and you know, they weren't wrong)....
....anyway, where was I...oh yes, the moral. I guess the moral of this story is don't trust Zeus. If you're a woman, he'll probably turn you into some kind of animal...and then fuck you....or turn himself into an animal...and fuck you...either way, he's going to fuck you. If you're a male, don't get in the way or he will have you killed...and then probably fuck you.