We have only 2 fish (a female platy and a male betta) and put them in the same tank for about an hour 6 weeks ago. We didn't leave them together long because it seemed to stress the betta out. We didn't think anything of it until I just found 3 babies in the platys tank. Not sure if she had more but we may have accidentally killed them when cleaning the tank last week. There's no other explanation but I can't find anything online about the two breeds making babies together.

We've had both fish for 6-7 months. The betta had never been in the same tank as another fish and the platy hasn't been in contact with other fish for about 3 months.