Currently Banned Characters/Species.

Temporarily Banned
Eevee
Zangoose
Fennekin
Treecko
Totodile
Mawile
Vulpix
Mareep
Ralts
Sneasel
Honedge
Archen
Aron

Restricted Access
-Zorua-
Zorua cannot actually transform into the Pokémon they copy, only disguise themselves and people have been known to use Illusion as an excuse to make their Zorua characters fly or breathe underwater. Due to issues with such a thing in the past, Zorua have been placed under restricted access. In order to make a Zorua, you can not have had issues with your characters in the past, such as characters doing things they're not supposed to do, and be a member, meaning have a light blue colored name rather than the darker blue.

-Mimikyu-
Mimikyu's appearance is virtually entirely undefined, and on top of that, it has somewhat of an issue tied to it in how "terrifying" it is described as.  The only barrier preventing someone from trying to reveal this appearance is a simple piece of cloth, which is a bit problematic.  Mimikyu has been put on restricted access due to the potential problems that could arise from giving the species out too freely.  The same restrictions apply as with zorua.

-Ditto-
Due to its inherent ability to turn into other pokemon, it should be obvious why ditto is on this list.  Transforming into others with their power is a bit hard to balance, so there are a few rules to follow with ditto.  Ditto characters get something similar to the version of Transform/Imposter present in the main-series games, except the ditto is not scaled up or down to the target's level, meaning it is a version of them at its own level.  That means no weak little fresh-made ditto character going around turning into a swole god.

-Smeargle-
Similar to ditto, smeargle is able to copy things.  In this case, it can permanently copy any move using Sketch, and this includes problematic moves like Transform.  The restrictions from the games on how and when it can copy moves are applied here, as well as what moves it can and can't use after copying.  Additionally, if a smeargle copies Transform, it can only use the PMD-version, in which the user takes on the target's appearance ONLY, and retains its own moves, ability and general power.

Permanently Banned
-Wailmer-
Its evolution is too big and immobile to use for the rp, the player would only be confined to one dungeon and is generally too inconvenient to have around.

-Phione-
Comes from manaphy which is a legendary, even with other phione as the parents.

-Rotom-
His forms wouldn't make sense in this world and wouldn't be accessible.

-Type-Null-
The lore does not support large numbers of this pokemon beyond as many as you can count on your fingers, therefor it is going to be kept away from being a playable character.

-Dracozolt, Arctozolt, Dracovish, Arctovish-
The nature of these pokemon not only fail to support larger numbers in a natural existence, but they are incapable of breeding-even with a ditto.

Current number of banned species: 11
Number until next reset: 20

I hope to gain just that little bit more diversity in our small Pokemon world. Sorry for any inconvenience.

As of March 3, 2015 banned species is to be counted based on how many people have that species rather than characters.

People can make more characters of a certain species that is banned if they have one of that species that they made themselves. You cannot make more of a species if it's a character you adopted though. So for example, Charmander is banned, but you adopted a charmander and want to make another that is your own, you will not be able to. However if you made one before they were banned, you are free to make as many as you please.

Additional info
These Pokemon are 1 away from being banned:
Pichu
Murkrow
Cubone
Sandshrew
Piplup
Riolu
Poochyena
Snivy
Absol
Minccino
Shinx
Chimchar
Shuppet
Cyndaquil
Espurr
Litwick

These Pokemon are 2 away from being banned:
Axew
Fletchling
Zorua
Growlithe
Oshawott
Croagunk
Mudkip
Charmander
Emolga
Spiritomb
Scyther
Purrloin
Houndour
Abra
Weedle
Gligar
Froakie
Snorunt
Sableye
Elgyem
Trapinch
Torchic
Bulbasaur