In training pack animals, consider the way each travels and also the ability of your string leader (the horse you ride) or pony horse to vary his speed to suit the pack animals behind him.
It's certainly better if all horses involved have similar gaits. A gaited animal may walk comfortably at a speed that would require trotting or even loping by a heavily muscled arena athlete, and neither the trot nor canter is good for the balance and security of your packs or for safety on most trails. Many mules, even non-gaited ones, are fast walkers.
But it's an imperfect world, and you may be training pack animals that simply can't keep up with the brisk walk of your pony horse. Then it's up to your string leader to slow down. Some gait training may be necessary.
All horses can be taught to walk slowly through gait training. This should begin with early training. Try for "cruise control." Your horse should stay in whatever gait you choose without either urging or restraining.
The horse that goes too fast can often be taught to slow down if you remember to apply quick pressure to the reins, then slacken them as a reward for slowing down. A steady, hard pressure for the hard-charger often just makes him worse. Hard cases may need to go back to basic training.
My two primary pony horses, Little Mack, still turbocharged at age 18, and Partner, my younger black gelding, slow down automatically when I pony another horse. Both will slow to a crawl when the horse behind them must pass over a downed log or through a creek. Both, too, have learned to quietly signal the horse behind that he's not to crowd too closely.
All this comes with time and experience, like anything else with horses.? (For more on training the pack animal, see Sketches from the Trail, The Trail Rider, November/December '11.)