Lucy and Sully ended up at the shelter around the same time, and as soon as they met, they became inseparable. When someone saw Lucy's picture online and came to adopt her, the two dogs made it very clear that it had to be both of them, or neither of them.
Both Lucy and Sully have fairly unknown backstories. Lucy came to the shelter as a stray, and Sully was adopted once and then returned to the shelter. Something about each dog drew them to each other, though, and Lucy became like a mother to Sully. He was just a puppy when he arrived at the shelter, and Lucy was a few years older.