I believe she's taken there by Crowley to protect the boys. She decides they're her pack, and she makes it her job to take care of them. She's invisible, so they don't know she's there for a long time, but she follows them around to keep an eye on things.

It's mostly (or entirely) told from her perspective, and she's confused as to why two males keep mating, don't they know they can't have pups, they really should find a female to mate with.

I can't remember the pairing, but it might be either dean / sam or one of them / cas, nor do I remember if Cas knew about the hellhound from the start, or if he came in later and made the boys aware of it.

Edit: Found, thank you! https://archiveofourown.org/works/11280039?view_full_work=true