Spurs are not tack per se, but rather an accessory a pony's rider or trainer can use to fine tune a pony's training. While spurs can be a fun addition to a ponyplay session, they can also be quite dangerous, and they have the potential to cause serious bodily injury if used in an improper manner.
For real horses (in English riding), spurs are used to lend precision to the riding aids, which is important in all equestrian disciplines. The spur lets the horse distinguish between two commands (think of typing with fingers (spurs) versus trying to type with your whole hand with an open palm (leg without spurs)). In ponyplay, this is usually not the case (though with some show/ridden ponies it can be); a rider uses spurs on a ponyboy or ponygirl to encourage the pony to go faster, or to punish or correct any misbehavior on the pony's part.
An important question to ask is: why do want spurs in ponyplay? If you're looking to occasionally get the attention of a pony whose mind wanders, you probably want a milder spur. On the other hand, if your pony enjoys a little pain, then you may want a harsher spur.
Let's start with a brief overview of what a spur is and what its basic parts are: A spur consists of a shank, neck and rowel. Many English spurs are thin, short, blunt and do not rotate, whereas western spurs tend to be heavier, longer and contain rowels that rotate.
Regardless of what you want the spur for, I would recommend starting off very slowly since spurs can cause serious injury. If you have never used spurs before, it can be very easy to misjudge the harshness of a spur and how much pressure is too much.
Points to remember when choosing spurs: