Burmy is an interesting new Bug-type pokémon which was introduced in Diamond and Pearl. It can evolve into 4 different forms, making it kinda like Eevee, but not. Just... not.

Each evolution adds a new type -- making it part Bug, part something-else -- and each form learns different moves. Also, the stats are distributed differently for each evolution, though the total remains the same (424)

A male Burmy will evolve into Mothim at level 20.

A female Burmy will evolve into one of three forms of Wormadam at level 20.

Burmy changes its form to match its environment after every battle it participates in. Which form burmy is in when it evolves determines which form the Wormadam will be.

There are three forms:

You get this form for fights in the grass
You get this form for fights in caves and on normal ground
You get this form for fights inside a building

Wormadam will be part Bug-type, and one of Grass/Ground/Steel as listed above. Mothim is Bug/Flying.

The steel form is the trickiest to get if you've beaten the game — most of the inside battles you fight are with gym leaders and team galactic members, which are one-time battles.

There are, however, a few places where you can repeatedly battle indoors:

The Elite Four
Jubilife TV Station
Every day, there will be a semi-random trainer in the top-left of the first floor who is willing to battle you.
The Old Chateau
Located in Eterna Forest. I hope you like Gastly.
Lost Tower
Located on route 209, below Solaceon Town. Again with the Gastly. (Also some Zubat).
Seven Stars Restaurant
Located on Valor Lakefront, the restaurant houses pairs of trainers who, once per day, will take you on in a double battle.
(More locations to be added as i come across them.)
Wormadam (Grass) Wormadam (Ground) Wormadam (Steel) Mothim
23Hidden PowerHidden PowerHidden PowerHidden Power
29Razor LeafRock BlastMirror ShotGust
32GrowthHardenMetal SoundPoisonpowder
41FlailFlailFlailSilver Wind
44AttractAttractAttractAir Slash