At the canter, your pony will have a "lead" (leading leg). This can be either the pony's right or left leg. It is common practice to change a pony's lead between strides without breaking gait (called a flying lead change) when changing directions, for example when cantering a figure 8, or if the pony will be cantering for a long stretch and the trainer wishes to avoid excessively fatiguing a single leg. However, in dressage a pony may be asked to change his lead more often as a show of his ability with the trainer asking the pony to change leads every four, three, two, or one stride.
Thus, tempi are just a series of flying lead changes. For a pony cantering by putting one leg forward, then bringing the other leg forward to meet it, a flying change would be to change the leg that is moving forward first. A cantering pony should always have the same foot leading, unless he is being asked for a flying change. When a pony's trainer asks for a flying lead change, the pony should then commence using the opposite foot to move forward first and continue using this new foot until again asked to change leads.
Tempi come in different beats: one tempis, two, threes, and fours. The number here refers to how often (in strides) the pony will change leads. One tempis mean that the pony changes his canter lead every stride; two means that the pony changes canter leads every other stride, and similarly for three and fours tempis.
For the human pony, tempi is not as difficult as it is for bio-equines, but your pony will likely have to practice the canter first to get good, energetic, strides, before he moves onto tempi. It is also useful to watch a bio-equine performing tempis if you can. A horse doing one tempis looks like he's skipping along.
"One man's wrong lead is another man's counter canter."