If you give to the human society, make sure your donation goes for pet care (vaccine, sterilization...) and not euthanasia or luxury items like new cars or furniture. I volunteer at a cat shelter and when we get big donations (for us 1K is enormous) we spend all of it very wisely and with frugality. With a big organisation like them, make sure they treat your money with the same care.

You need a lawyer for a reputable firm to set this up. He/She will act as your executor and will sell your assets, pay your debts (if you have some) and give what's left after taxes to them. It's a long process. it took my Mom over a year to take care of her Cousin assets (and she didn't have a lot of money).

Have you also thought of a scholarship fund for Vet school for underprivileged students? That could also be a great legacy.