It may not be a good idea to keep Boston Terriers off-leash, mainly due to their prey drive for small creatures. Their instincts may cause them to chase after rodents such as squirrels if they are off-leash in a park or other outdoor locations. Of course, you can train them to walk off-leash from an early stage. But their prey drive is unpredictable, so letting them loose without a leash is not recommended unless you know your locality very well inside-out.