I have strong suspicions that ramshorns can and do eat hydra. The few times I have had hydra infestations adding ramhorns seems to get rid of the hydra.
Unfortunately the ramshorns breed like crazy in my tanks, partly because of the live plants, so they always have that to munch on, and the MTS appear and chow down on the food meant for my corys and loaches. So far the Pakastani and Dwarf chain loaches I have are doing a great job of controlling the pest snails, but I can't put these loaches into all of my tanks due to the native snail species I want to keep and breed and the potential for the loaches to scoff my various types of shrimp.
But I have a plan, I hope that by getting various native snails (already have 3 types - two that will hopefully breed in fresh water), then these snails will eat the food in the tanks instead of the pest ones over populating.