“Some friends where at the beach and they found a half dead dog. They fed it and helped it get healthy and then one day they left it at home with the cats. When they returned there was blood everywhere and the cats were dead. They then found out that it was not a dog they found; it was a big Mexican rat.”
I have heard a few different versions of this story but they all end up with the dog being some kind of rap. The origin of this story is hard to find because this story has been told in so may different places. In some places they tell it a much different then this variant. For every different place there is a difference that may be different due to the culture in that place.