Thanks to my cousins for helping me out with this.

Name	Explanation
001. Bulbasaur	A kid's science experiment gone wrong. The kid tried to create an animal that was part plant by sewing a plant on a frog's back.
002. Ivysaur	The frog/plant combo grew.
003. Venusaur	It grew some more.
004. Charmander	A poor lizard got its tail lit on fire by a mean person.
005. Charmeleon	The lizard grew, but the fire wouldn't go out.
006. Charizard	The lizard decided to grow some wings so it could fan air onto its tail to make it go out.
007. Squirtle	A turtle held its breath too long and turned blue.
008. Wartortle	The turtle grew weird things on the sides of its head. It was too stupid to stop holding its breath, so it became a deeper colour of blue.
009. Blastoise	The turtle ate too many poké-snacks and got obese.
010. Caterpie	?
011. Metapod	?
012. Butterfree	?
013. Weedle	This bug was so stupid that they decided to superglue the dunce cap on its head.
014. Kakuna	?
015. Beedrill	A bee liked power tools so much, it turned partway into one.
016. Pidgey	?
017. Pidgeotto	?
018. Pidgeot	?
019. Rattata	?
020. Raticate	?
021. Spearow	?
022. Fearow	?
023. Ekans	A snake ate a bit too much and got chubby.
024. Arbok	The snake was too lazy to go on a diet, so it got 2 Machoke to stretch it out.
025. Pikachu	A mouse chewed on too many power lines.
026. Raichu	?
027. Sandshrew	?
028. Sandslash	?
029. Nidoran(female)	?
030. Nidorina	poison
031. Nidoqueen	?
032. Nidoran(male)	?
033. Nidorino	?
034. Nidoking	?
035. Clefairy	A wad of abandoned bubble gum that developed sentience.
036. Clefable	Someone pulled on Clefairy trying to get it off the bottom of the seat.
037. Vulpix	A fox's sister decided to try becoming a hairdresser.
038. Ninetales	A bad day at the dog groomer's.
039. Jigglypuff	A pink party balloon came to life.
040. Wigglytuff	The party balloon tried to look like a rabbit.
041. Zubat	Someone poked out a bat's eyes and cut off its feet.
042. Golbat	The bat grew them back.
043. Oddish	Yes, tennis balls do start growing if they've been lost in the garden long enough.
044. Gloom	?
045. Vileplume	?
046. Paras	A bug decided to grow his own food, but couldn't find a garden.
047. Parasect	A mushroom decided to grow its own seafood.
048. Venonat	It didn't run out from under my bed, honest!
049. Venomoth	?
050. Diglett	?
051. Dugtrio	?
052. Meowth	Some kid glued a coin to their cat's forehead.
053. Persian	When the cat and the kid both got older, they decided that it was tachy, so they replaced the gold with a ruby.
054. Psyduck	?
055. Golduck	?
056. Mankey	Primeape's little brother
057. Primeape	A big monkey got the worse end of a fight with a blow-dryer
058. Growlithe	?
059. Arcanine	?
060. Poliwag	?
061. Poliwhirl	?
062. Poliwrath	?
063. Abra	?
064. Kadabra	?
065. Alakazam	?
066. Machop	A kid who liked martial arts too much.
067. Machoke	The kid when he was older.
068. Machamp	The kid on steroids.
069. Bellsprout	?
070. Weepinbell	?
071. Victreebell	A pitcher plant had too much fertilizer.
072. Tentacool	?
073. Tentacruel	?
074. Geodude	A rock got tired of the same old scenery, so it grew arms so it could move around.
075. Graveler	?
076. Golem	?
077. Ponyta	A horse escaped from a barbecue party.
078. Rapidash	A unicorn escaped from a barbecue party.
079. Slowpoke	A sun-burned hippo.
080. Slowbro	The hippo got too close to a conch shell.
081. Magnemite	?
082. Magneton	?
083. Farfetch'd	?
084. Doduo	A bird was strangely mutated.
085. Dodrio	The bird was on steroids.
086. Seel	?
087. Dewgong	?
088. Grimer	I never knew those things under my bed had a name!
089. Muk	?
090. Shellder	A clam is shy, but sometimes rude.
091. Cloyster	?
092. Gastly	?
093. Haunter	?
094. Gengar	?
095. Onix	Geodude built a snake to keep him company.
096. Drowzee	?
097. Hypno	?
098. Krabby	?
099. Kingler	A crab on steroids.
100. Voltorb	a pokéball got caught in a lightning storm.
101. Electrode	?
102. Exeggcute	My little brother painted faces on the easter eggs.
103. Exeggutor	Someone painted faces on a palm tree.
104. Cubone	?
105. Marowak	?
106. Hitmonlee	?
107. Hitmonchan	?
108. Lickitung	The ancestors of the dogs who lick your face too much.
109. Koffing	?
110. Weezing	?
111. Rhyhorn	?
112. Rhydon	?
113. Chansey	Someone crossed a kangaroo with a pink party balloon.
114. Tangela	A small kid got trapped in the jungle vines, and came out like this.
115. Kangaskhan	An armor plated kangaroo.
116. Horsea	?
117. Seadra	A seahorse figured out how to use styling gel.
118. Goldeen	?
119. Seaking	?
120. Staryu	?
121. Starmie	?
122. Mr. Mime	The husband of the opera singer, also beaten. (see Jynx.)
123. Scyther	A weed-whacker gone mad.
124. Jynx	An opera singer who was severely beaten with an ugly stick.
125. Electabuzz	?
126. Magmar	?
127. Pinsir	?
128. Tauros	?
129. Magikarp	?
130. Gyarados	?
131. Lapras	?
132. Ditto	A melted mirror.
133. Eevee	A dog who shrunk, but whose fur didn't. That's why he's so poofy.
134. Vaporeon	The dog was drowned.
135. Jolteon	The dog stuck its tongue in a light socket.
136. Flareon	The poor dog was lit on fire.
137. Porygon	The nightmare of a student cramming for the geometry exam.
138. Omanite	A big sea snail.
139. Omastar	A big, angry sea snail.
140. Kabuto	?
141. Kabutops	?
142. Aerodactyl	?
143. Snorlax	?
144. Articuno	Your frozen chicken dinners have come back to haunt you.
145. Zapdos	A bird flew into the power lines once too many.
146. Moltres	A bird was set on fire.
147. Dratini	?
148. Dragonair	?
149. Dragonite	?
150. Mewtwo	?
151. Mew	A small cat was painted pink.

Of you were wondering how I came up with this, I was goofing around with my cousins and we made up these explanations. They suggested I put them on my page, so I did. There ya go.