How about sheep parts? A sheep attacked them, they killed it in one hit, and then the ranger skinned it, and put the skin, wool, and bones in his pack, and would have filled some vials with blood if there was any blood in the body. Unfortunately, having not planned for a sheep dissection, this led to the startling discovery of the lack of blood, leading them to guess that they were vampires. (That was a very weird adventure. Vampire sheep FTW!)