They are the only enemies in the Portal games, however they don't appear in Mari0, simply because Maurice doesn't like them, but they appear in Alesan's Entities. For more information about Turrets in the Portal series, please refer to the Portal Wiki.
Alesan's Entities Edit
The turrets will shoot Mario when they see them. They can be killed by having them upside down with a portal.
Mario will die if he is being shooted for a few seconds.