Small - Mice or Rats These are pretty quick animals so will be hard for the city defenders to catch/kill. They are pretty smart and will eat any food they find.They are also very good at surviving falls so are quite likely to survive landing, mice are more likely than a rat but rats will do more damage. With some training a swarm of mice or rats could be taught to go straight to the stores for food supplies which they would quite rapidly devour. They could also be trained to leave their waste in water supplies potentially poisoning the inhabitants. The main way they can help in terms of your question is that rodents have been observed chewing wood and wires so with training to encourage this behaviour they could destroy power supplies, pylons, and power flows in the city. With power supplies disrupted there would be no way to recharge communication devices and no lights at night making it easier to sneak in. It may also turn off the automated turrets which would be a big boost. 238
To deliver them all that is needed is to reduce the power of a catapult so it fires them quite gently as close to straight up as will still carry them over the wall. Fill this catapult with mice and you get a lot of mice per shot.
Medium - cats (preferably feral cats) They famously always land on their feet so landing shouldn't be a problem. Once they're in the city they can run around and start scratching/biting anyone they meet. Cat bites can easily become infected and so could cause many defenders to get ill, they probably won't die but it would put them temporarily out of action. The cats would also provide a decent distraction for any other attacks as well. Although you might need to train them not to eat the mice. I would suggest launching the cats in a similar flight path t the mice but only one cat per shot. Good luck loading the feral cat into a catapult.
Bigger than Cat- Not really worth the effort of firing them You could try launching badgers or moles, or even rabbits, to try and undermine the foundations of the defences but the amount of effort you would have to go to training them and equipping them with parachutes which would make them easy to spot and kill as they fall in. 160