I am trying to remove Animania from my server, but when I remove the mod, my server says :

10.11 12:39:41 [Server] Server thread/ERROR [FML.ModTracker]: This world was saved with mod animania which appears to be missing, things may not work well

And the server will not start. I really would like to get rid of the mod, there certain things we do on this server that won't work with out the animals (specifically chickens) works.

What exactly do you mean? Can you post the entire log?