From the Super Mario Wiki, the Mario encyclopedia
Thieving Slinkers are a rare variety of Slinkers appearing in ScareScraper mode in Luigi's Mansion 3, added in the version 1.3.1 update. They are dark-purple with glowing eyes and can steal Luigi's Poltergust G-00 similar to a ScareScraper trap that teleports the Poltergust G-00 to a random room. Once another player has captured the Thieving Slinker, the player will get their Poltergust G-00 back. They have 300 HP, more than double that of a regular Slinker. They are captured the same way as normal Slinkers.