You will need a "hex editor" and the hex codes of the animals you want. You can find the hex codes on the
A good, free hex editor is Ifinderlite.
Before starting I suggest you wait until it is night in your minecraft world and have as many zombies as possible follow you to the farm area. Make sure you have a safe place to jump up to.
1. Once the zombies have followed you to the farm area, quit the game to the title screen.
1. Open Ifinderlite
2. Locate your minecraft saved world
It should be in var/mobile/applications
3. Open the Minecraftpe folder then open
-documents>games>Com.mojang>worlds>"name of world you want to edit"
4. Open the entities.dat file.
5. On the right field there should be words. Look for the "id" line.
6.You should notice in the middle field there are numbers
Look for 6964xx the xx's being the mob id - this is what you want to edit.
7. Turn the xx into
0a - ducks/chicken
0b - cows
0c - pig
0d - sheep
20 - zombies
8. Repeat this step for ALL 696420 , this will turn all zombies into the animal of your choice.
I've noticed this method is a bit buggy. The animals will always spawn there now, even if killed.
If you use Intyre's impedit, this will reset the game to normal. The animals should still be there as long as you don't kill them. I am not completely sure on that last part, however.
I added an entity section to my MCPEedit that allows you to kill mobs instead of change them. All you do is transfer the entities.dat to your computer, open it with MCPEedit and a list of all the mobs shows up in the entities tag. Right click one of them and it kill it when you play the game.
The link is in my signature ↓