You are not strictly speaking guaranteed 1. With 8 boosters to a fat pack, there's about a 34% chance of getting no mythics.
Edited to add: If you want detailed percentages, here's some rounded to two significant digits:
Chance of 0: 34%
Chance of 1: 39%
Chance of 2: 20%
Chance of 3: 5.6%
Chance of 4: 1.0%
Chance of 5: 0.11%
Chance of 6: 0.0081%
Chance of 7: 0.00033%
Chance of 8: 0.000006%
Edited to add: Note that this does not include the possibility of getting a foil mythic.
In a given booster pack the mythic rare will replace the rare one eighth of the time. Note that this does not factor in premium (foil) cards, meaning that the odds are ever so slightly better than 1/8.
You are NOT guaranteed to get a mythic rare in a fat pack based on these statistics, unless WotC have specifically designed the packs as such to encourage buyers or something.
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I've never seen a fat pack in the Zendikar block that didn't have at least one mythic in it. The odds are actually as advertised in fat packs for mythic rares. On average, you'll find about 1 mythic in the 8 packs. It's not uncommon to get 2 though. If you really want mythics, I'd say fat packs are a decent way to get them. Better than buying 8 loose packs, anyways.
I've had two in every set since Alara Reborn, but I'm obviously an outlier. The majority of the time you'll likely see one, with another decent chunk having none. I've never heard of anyone having more than 3, and I've only heard three maybe twice.
I pulled three mythics in my Alara Reborn fat pack, one in M10, one in the first Zendikar fat pack I opened, and two in the second one. Obviously it's not a guarantee to get a mythic, but the odds aren't too bad. The part I hate is seeing two lonely fat packs sitting on a shelf and deciding which one to buy, then second guessing myself and wondering what was in the other one. I guess that's part of the allure of cracking open packs.
On Friday, I saw 16 ROE fat packs opened (my LGS ownere was somewhat happy). 13 of them had 3 or more Mythics in them. I can honestly say that I've never had less than 2 mythics from a fat pack... good mythics on the other hand...