I griefed my sister's creative world by making a really big lag zone. I spawned a bunch of NPCs using dispensers and eggs. Our PC is really bad so I couldn't go close enough to kill them in any way, lava, diamond sword, or TNT. The lag zone is so bad that we couldn't use a TNT cannon.
In addition, our dad doesn't allow us to download mods so don't mention downloading single player commands and do /killall.
Please note that they are NPC villagers that are causing a lag zone.
Best Answer
You could use McEdit to load your map and clean the world from those animals. This is not a mod but a tool that run outside the game so uninstall it after the work's done. Juste make sure you're not running the game while editing the world.
You could add lava where you think those animals are and put you in a safe place while watching them burns. When they're all dead, save the map, go in McEdit and remove the lava :D
Make a backup of your files before editing the map !