Wat r the M.O.M. classifications?

Well, the Ministry of Magic made these classifications to decide and tell others, how dangerous the magical cratures are. So, the ones with the high MOM classification XXXXX are very dangerous and know wizard-killers. You may be surprised about some of the magical 'creatures' because you may consider them near human, like Centaurs and Leprechauns, but both of these have decided to distance themselves from humans. The cassifications can be found in the book "Fantastic Beasts And Where To Find Them".
Hope I was helpful.