How do you get a fire mystic egg on the app Tiny Monsters?

Answer 1

First you need to buy the mystic cave, and fill up the orb in it to have a better chance of breeding one. Then you need to breed two pure breeds in the breeding den (ex, two fire monsters) until you come out with a mystic monster. You can do this with any kind of monster, including crossbreeds

Answer 2

First you need to have bought and built the Mythic cave. Then, keep breeding two fire monsters together (two of the pure fire monsters, no hybrids), eventually they'll make a mythic egg, but the chance is small so keep trying!