Sounds like a raccoon, but may have done you favor. Raccoons can kill fish, but they're not that good at it. Smelling a dead floater and taking it they're very good at.
More time spent learning about good pond care reduces depression. It can get complex but just learning the basics like water testing and pH buffering can get you 90% of the way. And a little bit about medical treatment, just enough to know it's beyond what you're will to do can reduce depression. I know a fair amount about medical treatment but I personally would never treat a fish because I know enough to know I probably can't fix them with my skill level and what lengths I'm willing to take. Instead I focus on water quality, easier to understand and have some control over.
Quarantining isn't needed with all new fish unless you're willing to go to the lengths needed to fix problems. If you were willing to treat a problem it's a lot easier to do in a hospital tank. Goes to what I was saying above...I wouldn't spent $20-50 and a lot of time to try and save $2 worth of feeder fish. It's a cruel world.
On the other hand, if you thought you wanted to treat fish in the future then this would be a good time to get a hospital tank set up and tested. Trying to do this later when there's a problem is just about impossible. High end fish keepers actually keep a hospital tank running all the time, keeping their less desirable fish there. That way if there's a problem you're ready to go. Again, not something I would do, but it's an option.
I wouldn't bother with buying "select" type goldfish, meaning the same varieties found in the feeder tank. It's common for pet stores to just pick out prettier fish from the feeder tank and place them into separate tanks and charge more. They are generally healthier, or at least there's no way to really tell beyond the obvious cloudy water, sluggish fish. Here's a good video on buying fish. Mostly about Koi, but much of it true for Goldfish too.
