During a Norse Lands campaign, on the second island, you'll find an area that has a stone archway with cats engraved on the archway's pillars. Here, you'll summon Freya's Chariot – a mount that requires 3 gems and 7 coins to activate. Once you have Freya's Chariot, you'll be able to make the cats that spawn around the island follow you. For those of you that have used the Stag mount in the base game, this works exactly the same. You can sprint so fast that the cats lose interest, so make sure you keep close.
Bring one of these cats to a farm, having upgraded it to have the mill house (which houses farmers at night). The farm will also need to be within your kingdom's walls in order for the cat to settle at the mill house. Once that's done, you'll be up 25 gamerscore. Not a particularly hard one to get if you know how to get it.
Credit to FaSCoRP for pointing out that the farm needs to be within your walls for the cat to be recruited.