It's a mystery: a new pond forms, where there was no pond before. Over time, it gets fish. Where do the fish come from? Flying fish jetting in from far off places? Fish materializing in the pond as though they had "Star Trek" style transporter beams? The true answers are a bit less whimsical, but no less interesting.
Break It Down To Three
There are three primary ways for fish to end up in new ponds. The first is that they are already there. The second is that they bring themselves. The third is that someone else brings them--usually humans. The dispersal of fish into new environments follows the same rules with fish as with any other life form: fish are just a bit more limited in their transportation techniques.
Fish Are Already There
There are two main instances where a new pond forms and fish are already there. In the first case, the pond forms as part of an existing water system: a dam is made (by men or beavers or natural events), and a pond forms. Or local flooding causes lakes and rivers to overrun their shores, emptying into new valleys and low lying land, creating new ponds when the flood waters pull back. In either case, the pond forms with water already part of an existing ecosystem, complete with algae, bugs and fish.
In the second situation, the pond forms in a region that suffers regular droughts. In this case, there are sometimes local species of fish that have adapted to survive the droughts by burrowing deep into the mud of a pond while it still has water and hibernating till the next rain floods the pond and fills it with water again. They then come out of hiding to feed and reproduce, continuing their life cycle until the next dry spell.
They Bring Themselves
There are other times when a new pond forms and the fish bring themselves. If the pond forms as a result of a spring, with a steady upsurge of water, the water may eventually spill over top the surrounding land at some point and create a creek, stream or river. If the creek connects with another body of water--another stream or river, a lake or the ocean--it creates a fish highway. Fish will move into the new territory, or migrate up the stream to spawn, and will eventually find their way to the new lake and populate it.
Far less frequent in much of the world, but not unheard of is for fish with limited ability to travel over land to hike from pond to pond. Species such as walking catfish spread their territory and have invaded any number of water systems since being released outside their native territories. This method is useful only to species that survive out of water, of course.
Someone Else Brings Them
While most fish will migrate in on existing waterways, there is always a chance of transport from other sources. A pond that forms near other ponds may receive new fish from passing birds of prey dropping their catch. Similarly, fish roe that remains damp enough during a trip between ponds may wash off of the fur and feet of local animals as they move from pond to pond.
The most common species to give fish a lift to new ecological niches, including new ponds, though, is mankind. Between the new bodies of water we seed intentionally, providing new bodies of water with valued sports species such as bass and trout, and the many times we accidentally release fish or fish roe into the new water by carrying them in on the mucky bottoms of our boats and other water gear. This also includes the times when loving aquarium owners empty illegal species into open bodies of water, mankind has proven to be a major cause of fish dispersal.
So there isn't that much mystery--just logic and science to explain fish in new ponds.