Yep, this was inspired by a conversation with my officemate, who wrote the original version. This is my take.

Gelatinous Cube Companion

You forged an alliance with an unusually aware gelatinous cube. Weird.
Prerequisite: Animal companion class feature (or ability that functions as animal companion class feature).
Benefit: You gain a gelatinous cube as an animal companion. It has base stats and an advancement at 4^th level, and special feats it can take. Despite being an ooze, it’s fairly intelligent and can learn tricks normally. Any spell you cast that has the (harmless) tag that would affect an animal can instead affect your gelatinous cube companion (though not any other ooze).

GELATONOUS CUBE COMPANION STARTING STATISTICS
Size Large (reach 5 feet); Speed 20 ft.; AC +1 natural armor; Attack slam (1d6, +1 point acid); Ability Scores Str 12, Dex 10, Con 18, Int 2, Wis 1, Cha 1; Special Qualities ooze traits (not mindless), blindsight 30 feet, determines carrying capacity as a quadruped.

4TH-LEVEL ADVANCEMENT
Speed 30 ft.; Ability Scores Con +4; Attack slam (1d6 + 1d6 acid) ; Special Qualities blindsight 60 feet.

Gelatinous Cube Companion Feats
A gelatinous cube companion can take the following feats when it gains feat options.

Acidic
Prerequisite: Gelatinous cube companion.
Benefit: The bonus damage from the cube’s slam attack increases by +1d6, plus and additional 1d6 for every 4 full HD of the cube.

Engulf
Prerequisite: Gelatinous cube companion.
Benefit: Although it moves slowly, as a standard action a gelatinous cube can simply engulf creatures of its size or smaller that are in its path. It cannot make a slam attack during a round in which it engulfs. The gelatinous cube merely has to move over the opponents, affecting as many as it can cover. Opponents can make attacks of opportunity against the cube, but if they do so they are not entitled to a saving throw. Those who do not attempt attacks of opportunity can attempt a Reflex save (DC 10 +1/2 cube HD + cube Strength bonus) to avoid being engulfed—on a success, they are pushed back or aside (opponent’s choice) as the cube moves forward. Engulfed creatures are subject to the cube’s paralysis (if any) and acid, gain the pinned condition, are in danger of suffocating, and are trapped within its body until they are no longer pinned.

Paralysis
Prerequisite: Engulf, gelatinous cube companion, 5 HD.
Benefit: The gelatinous cube secretes an anesthetizing slime. A target hit by a cube’s engulf attack must succeed on a Fortitude save (DC 10 +1/2 cube HD + cube Constitution bonus) or be paralyzed for 1d4 rounds. The cube can automatically engulf a paralyzed opponent.

Transparent
Prerequisite: Gelatinous cube companion.
Benefit: Due to its lack of coloration, the gelatinous cube is difficult to discern. A successful Perception check (DC 10 + cube’s HD) is required to notice a motionless gelatinous cube. Any creature that fails to notice a gelatinous cube and walks into it is automatically engulfed (if it has the engulf feat).

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