Five Pokemon

[Seki]

Ralts
Elekid
Pikachu
Lilipup
Yanmega

Next, SNAKE.

 

[何贤豪]

Seviper (A snake! xD)
Nidoking (Seviper and Nidoking are both Poison-types!)
Arcanine (And now for a change of pace...)
Kingler
Electivire

Okay...DEVIL.

 

[mishy.]

Darkrai
Electivire
Venipede
Ivysaur
Lanturn

GHOST

 

[RoreyG]

Gengar
Heracross
Onix
Serperior
Tyranitar


hmm..........JAMES (from Team Rocket)

 

[Altomare]

Jigglypuff
Aerodactyl
Mareep
Emboar
Shelgon

PERRY

 

[何贤豪]

Piplup
Eletivire
Raichu
Rotom
Yanmega

Perry the Platypus? Let's see...ENTEI.

 

[theanhvn]

Elekid
Nincanda
Tentacool
Emolga
Illumise

hmm....SURGE

 

[LegendaryMohawk]

Suicune
Unknown
Raikou
Gengar
Entei

And the 5 letters are: YXZAU (just to stop all the actual words people are doing :P)

 

[theanhvn]

Yanma
Xatu
Raichu
Alakazam
Ursaring

OXOXO

 

[LegendaryMohawk]

Onix
Xatu
Oddish
Xatu (it's the only pokemon starting with X...)
Oshawott

DDHAE

 

[mishy.]

Dragonite
Dratini
Haxorus
Axew
Emboar

so close with all dragons

KSUDF

 

[LegendaryMohawk]

Kakuna
Silcoon
Uxie
Diglet
Furret

YIOWG

 

[Altomare]

Yanmega
Ivysaur
Oshawott
Wailord
Goldeen

BICOL

 

[Correspondence]

Bronzong
Ivysaur
Charmander
Onix
Lilipup

Next is JSGDH.

 

[LegendaryMohawk]

Jynx
Scizor
Geodude
Ditto
Hoppip

DAFUQ

 

[CourageHound]

Darmanitan
Amoongus
Frillish
Ursaring
Quillava

Next up; SANTA

 

[Altomare]

Shelgon
Ariados
Natu
Tirtouga
Arceus

DWVFA

 

[AwesomeAsCrawdaunt]

Dragonair
Wailord
Vanillish
Ferrothorn
Alomomola

CRAWD

 

[Altomare]

Cradily
Rhyperior
Aerodactyl
Wailord
Diglett

STEVE

 

[AwesomeAsCrawdaunt]

Scolipede
Tepig
Emolga
Vespiquen
Empoleon

GORDO