"My tortoise was running."
Translation:Bha an sligeanach agam a' ruith.
There has been some discussion about this in various threads and it's simple in some ways but more complicated in others. The simple rule is that mo, do etc is for body parts and close family members and an ___ agam for other things, but then you run into exceptions.
The question of pets was discussed on one thread and these always seem to be an ___ agam, but one moderator did suggest that if you were particularly close to a pet you might use the mo form.
The basic explanation is in the tips and hints for the "Body 2" skill. If you're on a phone and you don't have links to the individual skill tips, they're all on one page on Duome. https://duome.eu/tips/en/gd