This Amethyst appears within My Little Universe: Mindwarp, as one of the threats encountering the Crystal Gems.
Appearance
She mirrors the exact same appearance as Crystal Gem Amethyst, the main difference being she's as tall and as muscular as Jasper.
Personality
She is shown to be very sadistic and violent, fairly similar to Rupee, as she was more than willing to just fight against her "friends" without much trouble, including her real self which she remarked that she could never defeat her.
History
My Little Universe: Mindwarp
As the Crystal Gems were traveling through the Dreamscape, and into its version of the Prime Kindergarten, Monster Amethyst first appeared and quickly confronted Pearl, demanding her to leave. She quickly started fighting her, taking great pleasure in doing so until she was found by the rest of the Crystal Gems. She tells them that this is her true form, her talking as if she was their Amethyst, and that shattering Gems is something she was made to do. The real Amethyst found her and started fighting her, the Monster Amethyst constantly berating her and saying she had no chance of defeating her. Amethyst quickly "destroyed" her by a combine attack of her whips.
It is revealed afterwards that this Amethyst represented Amethyst's own feelings about being a monster, and how the Monster Amethyst reminded her why she hated herself so much.