The Submarine Cats are a team of 2, or possibly more, cats who appear around the Duggart household every day. They live in a submarine off the coast of the island the Duggarts live on. They have not appeared in a very long time, but they may have sent their "Death Dog Probes" to spy on the Duggarts. In the picture here, a submarine cat transformed into the same type of skinny human robot form Waffles did in the past, possibly showing a connection.
The Submarine Cats have their first official agent appear in episode 116 of the Duggart playthrough. At first, the cat seems like an innocent stray sleeping in Gumbo's bed inside of the house, but once Robbie goes to pet the cat, it transforms and reveals itself as a Submarine Cat. Robbie runs from the house to escape, but Mageru and Oscar stay behind and don't really care. Two more members appear in episode 144, having apparently been observing the Duggarts and the many deaths that have been occurring.