The following Pokemon cannot be found in Pokemon Crystal. You'll have to transfer them from Gold, Silver, Red, Yellow or Blue. As always, Mew and Celebi can only be gotten from special events or contests.

Mankey
Primeape
Vulpix
Ninetales
Mareep
Flaaffy
Ampharos
Girafarig
Remoraid
Octillery
Bulbasaur
Ivysaur
Venusaur
Charmander
Charmeleon
Charizard
Squirtle
Wartortle
Blastoise
Omanyte
Omastar
Articuno
Kabuto
Kabutops
Zapdos
Moltres
Mewtwo