Grarrl is a Neopet, not a pokemon. So no, it can not be caught. Only Pokemon can be caught in Pokemon.
If you would like a Grarrl you can adopt one from the Neopets website.
*Mew is not catchable in Pokemon x and y, but you can get him online from wondertrades and such. Mewtwo however, is catchable and you can find him in the cave in pokemon village.
sudowoodo and smeargle
There is no official Pokemon game that has all Pokemon catchable in it, which means you'll have to trade and transfer Pokemon from other games to get all the Pokemon. However, some fan-made hacks do have all the Pokemon catchable in them.
yes, all Grarrl's eat anything and everything.
The catchable legendaries are the following: Articuno, Zapdos, Moltres, Raikou/Suicune/Entei *depends on your starter pokemon*, Mewtwo.
If you are talking about the 2 mascot Pokemon then you have the following: Reshiram - The "Vast" Pokemon. Reshiram is Fire/Dragon type and is only catchable in Pokemon Black. Zekrom - The "Deep" Pokemon. Zekrom is Electric/Dragon type and is only catchable in Pokemon White.
trade it from red,blue, or yellowMewtwo is not catchable in the wild in Pokemon Crystal.
i think it is machop.
The amount of available catchable legendary pokemon is three: Kyogre, Latias, Rayquaza.
look on bulbapedia numbers 152-251 national dex
Celebii is not catchable in Pokemon diamond. You have to transfer it in to Diamond from other versions that have Celebii in it.
its not catchable in red blue or yellow but you can glitch it up and get it.