Vermin Traps were built to prevent stoats, weasels and other vermin killing and eating the rabbits.
They channelled or tempted the vermin into a granite walled container, where the trap would be sprung, capturing the animal until the farmer came to deal with it.
There are many of them in the rabbit breeding areas of Dartmoor, and are difficult to find, I have photographed a few which are on waymarking dot com.
This excellent website gives diagrams of the workings of the vermin trap.
The Vermin trap is just below the cache, look for an oblong stone with three holes, notice under the stone the channel where the vermin were trapped.