The PCs in my homebrew campaign setting are finally to the point where a lone [cat](http://dndsrd.net/monstersAnimal.html#cat) is not an excessively dangerous foe, yet a clowder of cats *could* still worry them. However, my research efforts have failed me: I was certain there was a method of converting individual creatures into collectives, but I can't locate it.

Are there official rules for making cats (or chickens or dogs or ponies) into a swarm or something similar so I needn't run a bunch of individual creatures? Absent official rules, are there published or playtested homebrew means to do this?