This could be one giant or multiple smaller giants that spawn in the Marsh. They are more likely to spawn when you have lots of rot and manure in Spring or Summer, the same way Bearger spawns when you have more Beeboxes.
Rotflies are not incredibly strong, but they are fast and can apply a lot of nasty effects to your base/supplies.
Rotflies' attacks to you will cause any food you're carrying to have its 'days until spoil' reduced by 0.5
Rotflies disease (by touch) can be used on your ice boxes and slowly rot all the food. Using disease on bushes, saplings, grass, trees, or farms will cause them to become diseased. The effect does not come immediately, but after the Rotfly is 'infecting' the object for 5-10 seconds which can be interrupted by the player
Rotfly larva will spawn in the Marsh if you have lots of rot and/or manure stored in inventories and chests, and develop into Rotfly after 1 day if not killed. I think a good amount would be if you have over 100 combined rot and manure then they start spawning.
Possible Drops:
Rotgland - Can be used to make Rotsack: rots any perishable item in 0.5 days
Rotstone: Place in any fridge or inventory to slow rotting by 75%
Rotserum: Remove disease from any plants: 50 uses