Question: Does anyone know why there are armadillos running around inside Dracula's castle?
Answer: Pretty sure this qualifies as a mistake, just like the llamas in Troy. Armadillos only live in the Americas. Later on, they have opossums too! So the story about not wanting to show rats could very well be true. Nevetheless, if this isn't a mistake, I don't know what is.
Chosen answer: If memory serves, that version was filmed in Mexico and they used the same sets to film the Spanish version AT THE SAME TIME. English crew on days, Mexican crew at night. Being the desert there would be armadillos and I'm sure the crew thought they'd make good rats or something. Wouldn't you find some sort of exotic wildlife living in Castle Dracula?
Answer: Actually, in that time period, rats were deemed too "gross" to show so armadillos were substituted. I got this answer straight from David Skal, the noted horror film historian.
Answer: So he could seduce Mina slowly with his hypnotic eyes, Dr. Steward would have been in the way.