Gadgeteer voltage sensor that can measure up to 8000 volts?
Measured inside the fence-charger case?  Or you want hv probes out in the rain?

Easiest is to just buy a high-voltage probe for a voltmeter.  These are just a thousands-megs resistor in a heavy insulator.  Hook a 10meg resistor across your A/D inp, then put the HV probe between the A/D-inp and fence wire.  A/D-common goes to earth.  That reduces 8kv pulses to perhaps 8v or 0.8v.  For added safety, perhaps put a chain of several 1meg resistors in series with that wire from A/D to earth.  With the impedances you're dealing with, a chain of 1meg resistors is a good conductor.

But if lightning sometimes kills your fence charger, you're asking it to come in and zap your A/D block, or worse.