I was gonna put and RFID tag in my cats collar, cos I have a cat flap that is done though magnets but of course other cats have the same collar and its not unique to all cats.

RFID developers kits can be had pretty cheap although its not difficult to build your own reciever and connect it to some form of microcontroller to do the processing

slightly extreme and only really thought about when theres next doors cat in my house,

i2c