Zeus did wildshape into a bull a couple of times (notably to kidnap Europa), but he didn't sire the Minotaur.
It's Poseidon who's to blame for that monster (and for the Medusa, as well). And Minos.
Once upon a time, there was a King of Crete named Minos. He gave his name to his civilization, the Minoans, and to a mythological monster, the Minotaur.
Why a monster? Because it's his fault, the fault of his hubris, the fault of thinking he could cheat a god!
Minos' island country relied a lot on sea travels, for trading and pillaging and doing all the kinds of things people with boats did in these ancient times. So he thought it would be a good idea to gain the favors of the God of the Sea, Poseidon. Sounds quite like a plan, right?
So, Minos grabbed his cellphone, made Posy's number, and called.
"Hello there, Posy, old pal, it's me, Minos, remember? That bearded king on the island."
Dude, I know of 4873 bearded kings of an island in this dang sea. You could be a bit more precise. But nevermind, I'm a god, so I get you. What's up?
"Well, I though it would be great to burn you a big bull in offering to show you I'm your buddy, but I'm afraid I don't have any bull that's kewl enough for a god like you. So I wanted to check if it would be alright with you to sacrifice a not-so-great-looking bull 'cause I don't have any better in stock."
It would be insulting, mate. Let me propose you a deal -- I give you a bull, and then you sacrifice it to me. Sounds fair?
"So you give me a bull for me to give it to you, and then we're buddies? It's a done deal, dude!"
Alright, then. Look on the shore tomorrow.
And then Poseidon hanged the phone.
On the morrow, there was a miracle on the beach. Out of the sea foam rose a wonderful white bull, mighty and healthy, powerful and awe-inspiring. A celestial bull if there ever was one. (Or maybe was it axiomatic instead? Well, whatever.)
So glorious was the bovine that Minos thought it would be a shame to sacrifice it before the bull could impregnate the royal cowherd with its quasi-divine genetics.
So, Minos did what any greedy farmer king would do, and kept the axiomatic bull in his stables, and instead sacrificed one of his mundane bulls.
Of course, ol' Poseidon was not fooled by the subterfuge. Vexated and furious to be so humiliated by a mortal that pretended to be his friend, he plotted a clever and perverted revenge.
So you fell in love my my bull, hey? Let's see the look on your face when your darling Pasiphae fall, too! The money I'll make out of the Pay-Per-View of this will more than make up for this slight!
And after pronouncing those terrible words, which implied such awful, grandma-unsafe things, he took his voodoo doll of Queen Pasiphae and cast a spell out of Valar Production's only book on her. Immediately, Queen Pasy felt unnatural urges and wanted to get naughty with the beautiful bull. With the ingeniosity mankind has always shown for murdering other people or satisfying freaky fetishes, she tasked the famous engineer Daedalus of crafting for her a follow cow statue in which she could fit in for... You guess what.
Anyway, once's Pasy's
geased fantasy was fulfilled (and Posy got rich selling this piece of snuff movie to the other gods), (un)nature took its course and nine month later, King Minos was the (un)happy (adoptive) father of a baby that wasn't exactly human.
Once again, Daedalus was tasked with a royal job, this time creating a labyrinth in which the proof that Minos had been cuckholded by a bovine would be hidden forever.
And when Minos means forever, he means forever. No witness. Daedalus and his son Icarus where marrooned on a tiny island and left to rot, forgotten by the world. Being the clever man he is, Daedalus crafted artificial wings so his son and himself could fly away to a more civilized place, and Icarus enjoyed flying so much that he soared too high, and then... You know the story.
Anyway, Minos, not wanting to pay the bill for his bastard son's food, had the bright idea of vainquishing Athens and demanding for tribute that the city would sent every year some of its finest young maidens and ephebians for a little pizza-delivery trip in the labyrinth. One day, one of those conscripted lad, a certain Theseus, had the bright idea of wooing a smart girl known as Ariadne, who herself was Minos' non-bastard daughter. Well, thanks to Ariadne's thread, Theseus was able to find the way out, so he went with a sword, killed the Minotaur, went out of the maze, and rode to the sunset with Ariadne, and they lived happily thereafter, while Pr. Minos' Secret Moon Base autodestructed. Or something.
Next time, I'll tell you how the same Poseidon defiled a temple to Athena by raping the young Medusa inside, and how to punish her for her crime of letting herself be raped in her temple, Athena cursed her with serpentine hair and a so ugly face that just looking at it could get you petrified in horror. Remember that in these time, they hadn't yet invented morales for stories.