Welcome to Pokémon Answers. What would you like to know?

I don't think you can catch Mankey in Crystal. You most likely need to trade with Pokemon Yellow, Red and Blue.

This article here talks a bit more about it:

Ad blocker interference detected!

Wikia is a free-to-use site that makes money from advertising. We have a modified experience for viewers using ad blockers

Wikia is not accessible if you’ve made further modifications. Remove the custom ad blocker rule(s) and the page will load as expected.