Pale jelly, a j or j that has an engulf attack that causes illness and a difficulty rating appropriate to that. Some may be found on Juiblex’s level.

New monster “aballin”, from D&D. It is a mobile j-class monster that can appear as a pool of water when not actively moving, and can possibly appear as a water elemental when moving. They can eat organic items like gelatinous cubes, though perhaps not instantly like gelatinous cubes, instead dissolving them some time after engulfing them. They are incapable of climbing stairs, and typically sit dormant imitating a pool until a monster draws near, at which point they become active.

When attacking certain amorphous monsters (could be just puddings and gelatinous cubes, but also maybe jellies and other blobs) in melee, it is possible for your weapon to get stuck in the monster, yanking it out of your possession and putting it in the monster’s inventory. Unless, of course, the weapon is cursed.

A b-class monster which is a shapechanger and may transform into any P, b or j monster.

Merge the P and b monster classes, on the grounds that puddings and blobs are both classes of amorphous monsters that are sparsely populated. (Some attempts have been made to also merge the j monster class in.)