Strong impacts can knock it out of its shell. This Pokémon was born from mutated nanoparticles.
– Pokémon Moon Pokédex
It eats dust in the atmosphere. The composition of the dust determines the color of its core.
– Pokémon Sun Pokédex
Minior does not evolve from or into anything, but it has 2 main forms: The Meteor Form, which it assumes at > 50% HP, and Core Form, which it transforms into at =< 50% HP. Its unique ability, Shields Down, is what allows it to transform between the forms (either at the start of the battle, or at the end of the turn). Like most form-changing abilities, Shields Down cannot be copied or replaced in any way.
While in Meteor form, Minior is defensive, with 100 Def / Sp Def, and 60 in its other stats. It has lower stat total, but in this form, it's immune to status. In Core form, its attack and defense stats are swapped, and its speed changes to 100 as well, but it loses its immunity to status conditions. If it is affected by status in Core form, changing back to Meteor form will not cure its status. Its form change is kind of similar to that of Aegislash (ie switching between defensive and offensive focused stats), except it's based on HP rather than a move.
In Core form, there are additionally 7 different colours of core. Its Pokedex entry indicates that the colour is from the composition of dust it eats.
Its Pokedex entry also says that it cannot stay very long in Core form; without the protection of the shell, it will soon die if not put into a Pokeball. This is featured in one of the episodes of the anime; Miniors are shown to be quite fleeting.
It seems that most of the weight of Minior is in its shell. Its Core form only weighs 0.3 kg (0.7 lbs), which makes it the lightest Rock type Pokemon.