its a parasitic twin. i saw it on Weird True And Freaky but with a cow. its like a siamese twin but the twin is a partially-formed animal inside the cat, and 2 deformed legs sprout from the neck. The parasitic twin mooces off the dominant twin, and the only way to find out is an autopsy.
Creepy, huh? Kept me up at night after I saw it on TV!!