Fantasy immortality idea - Bind yourself to a flock of ravens, stealing their lifespans. They're very intelligent and loyal familiars, and while they usually die by 15 they can live over twice that. A decade or more could be stolen from each without any chance of hurting them.
You can absolutely do other animals and they have some advantages and disadvantages, but I think that Ravens are the best choice overall.
Domesticated animals - Dogs/cats, etc. Much lower lifespan than ravens and unless you turn them feral and just sort of hope that the bloodline continues (which you will be sabotaging with your lifespan theft) you'll have to take care of generations upon generations of short lived pets that you'll have to bury regularly.
Short lived wild animals - You could do this with ravens too, just set them wild once you're bonded to a couple families, but they lose the utility of familiars and are less stable. That being said, Ravens remain the ideal short lived choice for wild animals. I would also assume that there are benefits to keeping them nearby or keeping numbers low; otherwise your best choice would be to cast a very wide net and just bond to as many as possible until it would take an extinction level event to drop them.
Long lived wild animals - Crocodiles, aligators, elephants, etc. Main problem here is the vulnerability, since most animals that survive long periods in the wild tend to have lower rates of reproduction. Ravens can produce eggs very often in reasonable numbers provided food is sufficient.
Balance ideas if you're into that
Bonding requires individual connections; you may not have to become best friends with what you're bonding too, but you need to do something to each one.
Limited efficiency; you might only get a fraction of the years stolen. This could be a hard number or based on weight or something else.
