Who's that Pokémon!? rules added!

[Malake256]

It's flipping Jynx.

This Ballon Pokémon tends to be forgotten... and it doesn't evolve.

[Naesala]

It's flipping Jynx.

This Ballon Pokémon tends to be forgotten... and it doesn't evolve.

qwilfish

this Pokémon was once the heaviest pokémon.

[Moydow]

Depends whether or not you count glitch Pokémon. H Poké was over 6000lb. If not, Snorlax.

This Pokémon has volcanoes on its back.

[Nickno]

Depends whether or not you count glitch Pokémon. H Poké was over 6000lb. If not, Snorlax.

This Pokémon has volcanoes on its back.

Camerupt.

This Pokemon is the only legendary Pokemon that can be obtained through breeding.

[Moydow]

Phione.

This Pokémon is apparently based on a Japanese charm whose name literally translates to 'shiny shiny Buddhist priest'.

[Seritinajii]

Castform!

This Pokémon scares itself by its own crying so much that it eventually gets tired and falls asleep. Apparently is usually quiet, but can be as loud as a jet plane. o.o

[mtn_otter]

Whismur.

This Pokémon lived in jungles around 100 million years ago. Its skull is as hard as iron.

[Zenox]

Cranidos

This pokemon has the ability to send semi-trucks flying when on a rampage.

Edit:

This Pokémon is the mascot of a site who's title comes from it's Japanese name.

Also, for the record, the person who replied to this one was incorrect. It was Celebi and is the mascot for serebii.net. Just wanted to point that one out. 

[SoujiroElric]

Uhhh, Rhyhorn?

This pokemon can disguise itself, changing its appearance to that of any person or pokemon it can see. It can also show illusory landscape to disguise its nest.

[KidIcarus]

Hmmm... there's three that I think might be it.

I'm going to guess Kecleon.

[SoujiroElric]

Wrong, not Kecleon.

[Seritinajii]

Who is ...Latias? Because she turned into "Bianca" in that movie and probably at the end...

If so, then:

This pokémon goes out only at night for moonlight and stays buried in the ground in the day.

[SoujiroElric]

Who is ...Latias? Because she turned into "Bianca" in that movie and probably at the end...

If so, then:

This pokémon goes out only at night for moonlight and stays buried in the ground in the day.

Not Latias. Another hint: It's a dark-type.

Answering your question, it's Oddish.

[Seritinajii]

...is it a Honchkrow? I'm out of ideas.

[Malake256]

Zoroark...

[SoujiroElric]

Zoroark...

Ding ding ding!

Wow, I'm not going to ask about 5th gen pokemon anymore!

[Level 3]

This pokemon can effortlessly jump over skyscrapers.

[Moydow]

Blaziken, one of my personal favourites

This glitch Pokémon has a Featured Article on Wikipedia.

[Metechar]

Missingo?

This Pokémon lies standing still in the desert, and at night attacks thirsty, tired victims.

[SoujiroElric]

Cacturne, if not Cacnea.

This Pokémon can take down Indian Elephants with its electricity.