Who's that Pokémon

In a war, it’s difficult not to lose your head,
So take this, a fine piece of armour to be worn on your head.
But wait! Something is already inside,
Its high physical defence being the source of its pride.

It still has plenty of room to grow up,
But for now, it must remain a participant in the little cup.
Stuck there with nothing to do,
Until it can evolve and graduate to RU.

Don’t be bugged by its small stature,
It can be defeated so easily by a small amount of fire.
So when facing this creature, steel yourself and don’t fret,
You realise the thing you fear is nothing but a helmet?