Power box/transformer for the electric fence. I have one on our place hooked up to 3 miles of hotwire. Touch the wire and you get lit up!
Strange though, I don't see a 110 plug in attached to it for power. Also, I'm surprised it just sitting up on the post so that everyone can see. Likely to be stolen, they are worth about $75. BTW, those are Angus Cross Heifers (female) calves. These we would call "Baldies" due to the white on the face