Mobs generally spawn from dungeons, or spawners. Their natural spawn rate has been slowed down to prevent lag.

 

They usually spawn in the mining world right after it's been reset, this usually happens every 2-6 weeks.

Oh I forgot to mention, mobs only spawn in the Nether and in the Mining World.

 

Is there anyway to spawn them in the main world to farm?
(other than horses i know you can get eggs for those)

 

There is no way, main world has all mobs except horses disabled.

 

I've seen a few tabbycats in the mail world though...

 

That was when horses were first added and some cats spawned, not many though.

 

sthegreat2 and also, you can't breed animals

 

Why not?? :(:( im sad now.

 

Well if you were able to breed animals then you could do mass production of a lot of mob items. This would make that market (the one of mob items) go down or be unstable. In order to keep a balance this is actually a very good measure to be taken.
The best way to obtain mob items now is with a Donation Feature: ext creations (please someone correct me if I'm wrong here).
Another reason is lag.. Imaging if everyone was to breed 100... No, 500+ animals just to produce a lot of items. The lag in the server would be unbearable.