It is purely luck. But your chances are affected by two things you can influence:
Your characters Luck Stat and the amount of Floors you have traveled in a your current run.
Both of these increases the spawn chance of a rare enemey.
thanks, magic flute also spawns a rare enemy, just crafted it when doing a run, now i got the sheep tf, how do i get pebbles for the other tf?
You can get them from Mining Spots in the Magma Cavern Biome.
