If there has only ever been one loach in this tank (until now), the fry must have come in with something, either as an egg or a tiny fry in plants, wood, etc. Loaches are cyprinids and egg layers, and the female lays the eggs and a male must be present to fertilize them when they are being laid.
Loaches are very highly social fish and must be in a group of the species. Five is minimum. Now, I don't know the tank size, and Botia striata is not a large species, but it does need a group of the lone fish will be stressed.