Gail is probably right on target. Goldies can reproduce at quite a
young age. Your first ones may have had some fries, especially if
there were not a lot of other fish around to gobble up the eggs or
fry. The new fry argue that you have two breeding goldies in your
pond. If I had to vote, I would vote for your original and one of the
black mystery fish.

We have had two black/bronze fish for three years. One of them
finally began to change color this year. The dark color of your
surprise adults suggests that they are young as well. Perhaps
survivors from the first batch from last year's three goldfish.

Once there are a good number of adults, they will limit the number of
survivors from spawns.

Have fun with the mobs of fish.

Jim