There are two mods out there that you could've found if you simply used the search function. Ayutashi's Spawner mod(I forget the exact name, but it has the word spawner in it), and Risugami's Spawner GUI mod.
1. In the Application support on your computer (windows is easy to get to, just go to "Run" then type in %appdata% then find .minecraft and open it) In there, there should be a TMI text document. Open it and scroll down to the bottom. There is something about changing the mob that spawns. Change it into what ever mob you want to spawn.
2. Get Not Enough Items. You need mod loader. In there it's like TMI but you can chose whatever mob spawner you want.