I have recently started a little one-on-one campaign with my wife and have been inspired to homebrew some monsters that make use of the pain and pleasure mechanics! I've decided to post them here for anyone who's interested!

The Fliger is a large, tiger-like monstrosity that likes to abduct individual creatures and carry them off to a secluded lair, where it takes sadistic pleasure in dealing as much pain on the victim until its body or mind gives out. It has three lashing tails, each tipped with a different painful implement.

This one is partly based on the Laticifex oozes, but I gave it some gelatinous cube vibes so that you won't know at first that it's a threat, and emphasized the pleasure mechanic.

The Pain Eater is a two-headed, quadrupedal predator with no eyes but very sensitive ears. It has natural psychic ability, which it uses to link the physical sensations of the victims it holds in its rows of needle-sharp teeth.