Here you will find all the pokémon that are either only in the red version of the game, or the blue version. There are also pokémon that evolve only after being traded. The HTML is screwed up at the bottom of the page.

Red only pokémon
#125. Electabuzz
#123. Scyther
#023. Ekans
#024. Arbok
#056. Mankey
#057. Primeape
#043. Oddish
#044. Gloom
#045. Vileplume
#058. Growlithe
#059. Arcanine

Electabuzz and Scyther do not appear at all in the blue version, while the others can appear in duels against other trainers.

Blue only pokémon
#127. Pinsir
#126. Magmar
#052. Meowth
#053. Persian
#027. Sandshrew
#028. Sandslash
#069. Bellsprout
#070. Weepinbell
#071. Victreebell
#037. Vulpix
#038. Ninetales

Magmar and Pinsir do not appear at all in the red version, while the others can appear in duels against other trainers.

Evolutionary Link
#064. Kadabra evolves into #065 Alakazam
#067. Machoke evolves into #068 Machamp
#075. Graveler evolves into #076 Golem
#093. Haunter evolves into #064 Gengar