Speaking of Mew, I always wondered why it isn't Normal type. Since it is the original Pokemon, the generic, unspecialized Normal type honestly makes much more sense than the Psychic type, which is a specialized type.
Psychic-types have sort of been used as the "mysticism type" for a long time before Fairy-types came into the picture. Normal-type gives the idea there's no extraordinary power they have or it has some sort of neutrality to them if they were only part Normal-type. Mew is very special and though maybe could have been Normal-type at one time in the past it could be that the Mew living nowadays have themselves evolved to being pure Psychic-type as they no longer need to evolve into other Pokemon thus were able to hone their mystical powers.