In-pond filter and pump system
The in-pond filter kind of does exactly what they say on the tin, the pump and the filter both sit within the pond water and water is sucked through the filter into the pump and then straight back into the pond. These are used for maybe a small fountain or other features and general pond circulation.
I have never used these before but I imagine they could be a pain to clean as you would need to drag it out of the pond each time and would have limited capacity before the filter gets blocked or full.
You could maybe have one of these as well as a more traditional filter so that the water is kind of pre filtered before it leaves the pond and goes into the gravity return filter or pressurised filter.