An electrified butt plug is simply a butt plug with electrodes attached to its surface (or a metal butt plug where the entire plug is used as an electrode). The electrodes are attached to a (usually battery powered) controller on the pony which can be activated either by remote control (most common) or by a sensor on the pony (e.g. modified anti-bark collar to prevent the pony from speaking) or both. These can be used to provide pleasure or pain (or both).
Butt plugs with electrodes are fairly common and you can buy them at more than a few BDSM retailers (see partial list at bottom of page). However keep in mind that you will also have to purchase a power supply for the plug (though such power supplies are also quite common and can usually be purchased from the same place you buy the plug), and most electrified butt plugs are not easily made into pony tails (i.e. it's hard to add hair) due to the positioning of the external electrical connections.
I don't like wearing butt plugs, but I will make an exception for electrified butt plug tails because they can be so incredibly pleasurable. When I'm wearing one of these I'm always on my best behavior so my trainer will reward me. Of course, the punishments that can be dished out by a more intense shock in the ass also play a role.
In ponyplay electrified butt plugs are often added into a tail (or a tail is added into an electrified butt plug). If the bulb of the butt plug is large enough, the battery and controller could be completely contained inside the plug and activated with a remote. This provides a much "cleaner" look than having wires trailing out of the ass, but you will be limited on the options of the electronics and you will have to make this yourself.
Electrified butt plugs are very effective in controlling a pony, second only to electrified urethral rods in my opinion. Obviously the settings of the electronics can make the stimulation very pleasant (think electroejaculation) or very painful (think cattle prod shoved up the ass).
Retailers of Electrified Butt Plugs
Below I've listed a few retailers of electrified butt plugs and power sources. However, these are not electrified butt plug tails. At this time (2014-12-21), I am not aware of any retailer that sells an electrified butt plug that is also a ponyplay tail. You will likely have to do some tweaking of one of the below plugs if you want a combined tail/electric plug. If I come across a commercially available combined electrified butt plug ponyplay tail, I will definitely post the link below.
You should keep in mind that you will need to have a power supply (e.g. the Electro-Stimulation P.E.S. Power Box or the Eros Tek ET232) to deliver power to electrified butt plug. This is true of most other electrical toys (e.g. electrified butt plugs, electrified cock rings, an electrified speculum, etc.). While you will only need one power supply that you can use with multiple toys, power supplies are relatively expensive (~ $200 - $500 USD).