This is an idea I just had, to list out all the Pokemon that have been featured in the anime so far. It's an interesting project, plus it allows us to see at a glance which ones have been shown already and predict which ones will appear in future episodes. Just because a Pokemon has been featured already doesn't mean it won't play a key role later on (Grovyle and Torkoal being obvious examples), but we're still more likely to see Pokemon that haven't been featured yet as opposed to ones that have. For the purposes of this list, I define "featured" as Ash or May having checked the Pokedex on that Pokemon, or at least that Pokemon having had a significant amount of screen time.

The Pokemon in bold and dark red have been featured, those not in bold or dark red have not. And this only refers to the Hoenn Pokemon, since just about all the others have already been featured, and Hoenn Pokemon are obviously the focus now. Also, this list does not include Pokemon featured in movies, since those still usually get a feature episode in the main anime. That may not be the case for some of the legendaries though, so I will consider legendaries from movies as having already been featured.

Absol
Aggron
Altaria
Anorith
Armaldo
Aron
Azurill
Bagon
Baltoy
Banette
Barboach
Beautifly
Beldum
Blaziken
Breloom
Cacnea
Cacturne
Camerupt
Carvanha
Cascoon
Castform
Chimecho
Clamperl
Claydol
Combusken
Corphish
Cradily
Crawdaunt
Delcatty
Dusclops
Duskull
Dustox
Electrike
Exploud
Feebas
Flygon
Gardevoir
Glalie
Gorebyss
Groudon
Grovyle
Grumpig
Gulpin
Hariyama
Huntail
Illumise
Kecleon
Kirlia
Kyogre
Lairon
Latias
Latios
Lileep
Linoone
Lombre
Lotad
Loudred
Ludicolo
Lunatone
Luvdisc
Makuhita
Manectric
Marshtomp
Masquerain
Mawile
Medicham
Meditite
Metagross
Metang
Mightyena
Milotic
Minun
Mudkip
Nincada
Ninjask
Nosepass
Numel
Nuzleaf
Pelipper
Plusle
Poochyena
Ralts
Rayquaza
Regice
Regirock
Registeel
Relicanth
Roselia
Sableye
Salamence
Sceptile
Sealeo
Seedot
Seviper
Sharpedo
Shedinja
Shelgon
Shiftry
Shroomish
Shuppet
Silcoon
Skitty
Slaking
Slakoth
Snorunt
Solrock
Spheal
Spinda
Spoink
Surskit
Swablu
Swalot
Swampert
Swellow
Taillow
Torchic
Torkoal
Trapinch
Treecko
Tropius
Vibrava
Vigoroth
Volbeat
Wailmer
Wailord
Walrein
Whiscash
Whismur
Wingull
Wurmple
Wynaut
Zangoose
Zigzagoon

Feel free to point out any mistakes.