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Bill evolves from Billy at level 37. Billy was only obtainable from the 2015 Halloween event, along with Dopplegengar and Chompy, and (at the time of writing this) is the only Pokemon from the 2015 Halloween event that has not returned as an obtainable Pokemon for another event. During the 2015 Halloween event, Billy had a random chance of spawning at the small platform where Mew and Meloetta spawns at in the Mew Tree (at the time, normal Pokemon weren't obtained through requirements, but instead, were found around the map like a much more common Legendary, though it wouldn't be announced if they spawned). Bill is incredibly rare to see nowadays, as not only was the game not as popular before as it is now, but also, there was a massive data wipe later that year in December, which may have resulted in many losing Bill. As for Mega Bill, there was originally an exclusive mega stone for Bill which would have probably been a tournament prize, but now, you simply need the Slowbronite to mega evolve Bill. Bill is one of the only Halloween event Pokemon that can mega evolve, with the only other being Skeletoise.