Harvest power from an electric fence
Those fence chargers usually generate a high voltage spike on the wire fence, relative to ground. The wire fence is on insulators to make sure that almost no current flows, unless something touches it. I've heard that wet grass leaning on them can draw enough current to effectively neutralize such a fence.

So here's the take-away: there isn't so much a current through the wire as there is a voltage on the wire (capacitive charging currents neglected). So if you connected one end of the high-voltage side of a flyback transformer to the fence, and the other end of same winding to ground, you ought to be able to pick off slugs of current on the low-voltage side of the transformer, at a reduced voltage. 

Caveat if you try this with a flyback liberated from an old CRT: they often embed a diode in series with the HV winding in those units, and they're usually potted, too.