I wanted a Metang, and so did my brother. BUT! When I tried, I saw the Metang totally bust my collection of balls. But why? It is not legendary---DESPITE THAT CATCH RATE.
It has a very low catch rate at three. This is lower than many legendaries and all non legendaries with a chance of .4% of a chance to catch it at full health with a pokeball. Its just how gamefreak wanted it but the probable reason is that it evolves into a psuedo legendary so gamefreak wanted to make it a challenge to get metagross who is very good because of its psuedo legendary status(although the other psuedo legendaries dont have this catch rate). Source: http://bulbapedia.bulbagarden.net/wiki/Metang