This creature appears to be a yellowish-brown amoeba.
AC 17, touch 9, flat-footed 17 (+8 natural, -1 size)
Speed 30 ft.
Str 15, Dex 10, Con 22, Int 10, Wis 10, Cha 10
A mustard jelly secretes a digestive acid that dissolves only flesh. Any melee hit deals acid damage.
A mustard jelly can, as a standard action, split itself into two identical jellies, each with half of the original’s current hit points (round down). A jelly with 10 hit points or less cannot divide itself. When divided, each jelly moves faster than the original (base speed 40 feet per round).
A mustard jelly is immune to electrical effects and magic missiles. If targeted by an electricity effect (including area effects) or a magic missile spell, the mustard jelly gains temporary hit points equal to the amount of damage it would have otherwise sustained. These temporary hit points last for 1 hour.
The mustard jelly exudes an aura of poison in a 10-foot radius centered on it that slows (as the slow spell) any creature in the area as long as it remains in the area and for 1d4 rounds afterward if it fails a DC 21 Fortitude save. A new save must be made each round a creature is within 10 feet of the mustard jelly. The save DC is Constitution-based.
Environment temperate marshes
Mustard jelly appears to be a yellowish-brown form of the ochre jelly and is thought to be a distant relative of said creature. However, the mustard jelly is far more dangerous than its supposed relative because it is intelligent.
The mustard jelly gives off a faint odor of mustard plants to a range of 20 feet. Though it possesses intelligence, a mustard jelly cannot speak.
A mustard jelly attacks by forming a pseudopod from its body and either slashing or enveloping its foes. Mustard jellies prefer to attack from ambush or where they have the upper hand on an opponent. If combat goes against a mustard jelly, it does not hesitate to flee, though often it uses this tactic to lure unsuspecting foes in closer.
Mustard Jelly from the Tome of Horrors Complete, Copyright 2011, Necromancer Games, Inc., published and distributed by Frog God Games; Author Scott Greene, based on original material by Gary Gygax.