Beagles can be bred down to sizes much smaller than is safe, by the process of breeding together "runt" dogs. It is not uncommon for a litter to contain at least one dog that falls to the lower end of the standard, 20 lbs. (9.07 kg) fully grown.
Those hoping to produce a so-called Pocket Beagle will then pair such a small dog (referred to as the runt of the litter) to another runt of the litter. In some cases, since finding such small dogs is not an easy task, inbreeding is done.
The method of inbreeding is a controversial one, since this means that mother is paired with son, brother to sister and so on. The goal is to produce smaller than natural dogs and sadly, this means dogs that can suffer from a host of medical issues.
Since a "runt" dog can be susceptible to many health conditions, this takes those health issues and pushes them down the breeding line. This is unethical, to say the least.
In other cases, a Beagle is bred with another toy or small dog breed. In some cases, the puppy will resemble a Beagle, but will have the genes of a smaller dog breed; thus making the dog tiny. When this happen, sometimes, the dog will appear to be a Beagle while he is a young puppy, but as the dog matures, features of the other bred begin to emerge and it is only then that owners may suspect that this method was done.
When a Pocket Beagle is produced in this way, it could be that the ears do not drop correctly, a longer coat grows in, the muzzle appears narrow or other body structure elements that make it more and more obvious as the pup grows into an adult.