Without starting up my game, I would hazard the guess that it would be the Fly. You can easily catch one by letting a bunch (the smallest possible, of course
) of turnips rot. The fly will either be sitting on the turnips or buzz around. If you don't see it, just move away and return, it will show up eventually. Please be sure you have your gates closed, with gates open the Fly won't appear. Good luck catching your last bug!