While most of my ponyplay scenes (at least casual scenes) involve simple rubber bits gags (straight mouth rubber snaffles), there are times when my mouth craves a harsh bit. Of course, there are also times when my trainer may use them for punishment whether I feel up to them or not.
One of the harshest bits I've found that is still widely available from many tack stores (though I would never use one on any bio-equine), is the bike chain bit (photo at right). It is just like it sounds, a length of bike chain is used for the mouth piece (and notice the direction the serrations are pointing).
Spiked bits are extremely harsh and should be used with great care. They are, aside from bits made with (or wrapped in) barbed wire, the harshest bits I am aware of.
However, when used very carefully, spiked bits can be great fun if you like more than a little bit of pain. A pony with a spiked bit in its mouth is a very responsive and obedient pony. The photo below right is me wearing a bit with a spiked mouth piece. The spikes on this bit are not especially sharp, but they're more than enough for any reins pressure to get - and keep - my attention.
Spiked bits are somewhat hard to come by (which hopefully means they are not being used on bio-horses), but they are out there. An alternative to the spiked metal bits are the "spiked rubber bit gags" that can be found at some online BDSM retailers. These are relatively mild but will still provide a more intense effect than an ordinary bit gag.
If you are interested in the more intense spiked metal bit shown in the photo at the right, keep in mind that these bits are not nearly so mild as the bits you will find in fetish shops. The metal versions will really mess up your mouth, teeth, and gums, and they can cause bleeding if you're not careful (and sometimes even if you are careful). Even if the metal spikes are not needle-sharp, they can still do damage.
Thin twisted wire bits are also quite harsh and can really cut into the corners of the pony's mouth. These bits aren't quite as bad as bits with non-blunt spikes on the mouthpiece or bits with a bike chain mouthpiece, but they can be more severe than blunt spiked bits if the wire is especially thin and/or has a nasty twist in it.
Moreover, if wire bits are fastened tightly in the mouth, they can rapidly become painful even without any rein pressure. Below is an example of a bit with a double wire mouthpiece where both twisted wires are reasonably thick:
Here is an example of a bit with a relatively thick wire mouthpiece (not as severe as a thin wire, but more severe than the double wire mouthpiece shown above):
As mentioned above, the severity of a wire bit is increased if a twist is added to the wire: in the most extreme scenario, the twist is thin and very pronounced, like the threads on a screw.
A step down from the thin wire mouthpiece is the chain mouthpiece. A bit with a chain mouthpiece is not quite as severe as the bike chain mouthpiece, but it too can be really hard on the corners of the mouth when fastened tightly or when any rein pressure is applied.
The spade or spoon type mouthpiece can be a severe bit. These are typically seen on Western curb bits (bits that work via leverage), where the mouthpiece can put pressure on the roof of the mouth when the reins are pulled. My main show bit (for me, not any of my bio-horses!) is based on a spoon type mouthpiece with addition of an attachment point for my tongue piercing to add to the severity.
Below is an example of a western bit with a spoon mouthpiece, but you can a similar type of mouthpiece on english bits (my version of this bit is english, and is a Pelham).
Bits with rubber tongue ports (also called tongue depressors) can be severe in the sense that they can stimulate the gag reflex. The tongue curb can be either integrated into the bit or attached separately. However, a metal bit with a built in metal tongue curb can be harsh when the curb reins are used since this will really bite into the soft palate. I've worn one before, and I really like it.
There are of course many, many more horse bits that are harsh, but this is probably enough for one article. I will put up an article on making bit that attaches to one more tongue piercings to keep the tongue in place. I think this addition adds a lot to the sense of being controlled.
One final note: you can easily change mouthpieces on some types of english and western bits. This way you can change from spade/spoon bits, to rubber straight mouth, etc. This will save you money since you only have to buy the mouthpiece.