I had an apartment in college that allowed animals. They sold the apartments and the new owners had us come in to resign updated contracts. They wanted everyone with animals to pay and extra $100 a month (per animal) or get rid of the animals. Their thoughts were that all the tenants were just kids so we'd all just get rid of the animals. Not true. We all (those with pets) packed up and moved. Plus they couldn't penalize us for braking the lease because of the situation.
I don't think its a good idea to sneak animals in where they are not allowed. The animals should come first. That being said- I know some land lords who state 'no animals' but don't care if their tenants do have them.