Perthshire Bloodsucker

I've finally managed to track down the story I referenced in my previous post, regarding a possible sheep mutilation in Perthshire. I had it in my head that it occurred in the general vicinity of Loch Tay, but actually it occurred further North, north west of Glen Tilt, hence my inability to find it in my copy of Geoff Holder's Guide To Mysterious Perthshire, from which this account is taken:
The story takes place around 1923, in an old bothy to the north west of Glen Tilt. Two poachers decided they were going to stay the night in this remote location. The door was locked so they broke a window to gain entry and lit a fire. One of the poachers climbed back out through the window to get some water when something attached itself to his leg and began drinking his blood. He broke free and he and his companion searched the area to try and find what had attacked him. They could see some white winged objects and some faint blue lights. Somewhat perturbed, they forced the door of the bothy, cooked a meal and didn't sleep a wink.
In the morning, the only physical traces they could find were their own footprints, though the poacher carried the scars for the rest of his life. They never returned to the area. The bothy had an evil reputation among locals, but it's not clear whether this was a result of this incident or whether there were other tales attached to it.
Obviously, it's difficult to know what to make of a story like this. However, it does sound reminiscent of accounts such as The Skinwalker Ranch where small balls of light were associated with livestock mutilation.
Could whatever these things are got fed up of sheep and fancied a bit of poacher? Or, maybe more likely, did a wily gamekeeper spread a tall tale to deter poaching on their patch?
As an aside, Geoff Holder's written a whole series of guides to the mysteries of various Scottish counties, including Perthshire, Stirlingshire and Invernesshire, which I highly recommend. They cover everything from local myths to ancient sites to curious buidling and modern UFO and creature encounters. Invaluable for the Scottish Fortean (or Forteans with an interest in Scotland).