[MMM]

It is very common for katakana words in Japanese to be shortened this way.

For example, Personal Computer is パソコン (Pasocon)

Word Processor is ワープロ　(Waapuro)

Pocket Monster is broken down the same way in Pokemon.

ポケットモンスター is the official name for Pokemon, so the name was not "changed" but just put in an easier to say format, just like "Digimon" is short for Digital Monsters (デジタルモンスター).