When you trade rat meat to the goblin in the Gobsquat, if you have cat skins in your inventory, he takes those also. He gives you the same 20 gold and 5 xp each that the trader in the Hanged Goblin would. They do not count toward your total number of rats traded in as far as he is concerned, so it is not exploitable, just odd.
That dirty goblin is trying to run the pelt trader out of business!
Snoty snot.