
I tend to use animal parts (and humanely only I stress, I would never kill anything at all) to represent the ethos of the animal in my workings. So I might use snake skin for a spell that required stealth for example. I might use fox hair for a spell that needs cunning or cat hair for a spell for intelligence. No idea at all what I would use any part of a degu for, I suppose if I ever need to do a spell to make somebody really really irritating then a bit of degu hair might be useful....
I don't use animal blood but that is mainly because I can't think of any humane way to extract it. And it isn't too great when its all congealed. Hair is a much better prospect.
If I can't use the animal itself then I will meditate on the nature of the animal and draw its spirit and characteristics into my circle. Or a pictoral representation can work quite well. Its all about knowing what the animal can offer and thinking of a way to release that energy for your workings.