Pidgey:
Pigeon + Budgie
Pidgeotto:
Pigeon stuff
Pidgeot:
Pigeon with the word Jet phonetically incoroprated into the name
Rattata:
Rat + TaTa which is the general sound made by a small rat
Raticate:
Rat + Irraticate (To destroy)
Spearow:
Spear (weapon) + Sparrow (bird)
Fearow:
Fear + ow (from Sparrow)
Ekans:
Snake Backwards
Arbok:
Kobra backwards
Pikachu:
Japanese translation of Lightning Mouse
Raichu:
Japanese Translation of Thunder Rat
Sandshrew:
Sand + Shrew (small mouse like animal)
Sandslash:
Sand + Slash
Nidoran - Nidoking
All start with Nido (Phonetic sounding like needle) plus the suffix of the word.
Nidoran: Ran (Running related)
Nidorina/o: Refering to a ballerina for Nidorina, and just the masculine form for Nidorino
Nidoqueen: Queen
Nidoking: King
Clefairy:
Referring to the Clef from a musical standpoint, and then fairy.
Clefable:
Clef from musical standpoint + Fable (Tall Tale, Story)