Original Ben 10 aliens: Starlequin and Fragon by MisakaLovesYou on

Rath by KungFuMikey77 on DeviantArt

Ben 10 hero time. Stinkfly by mineSonic06 on DeviantArt

Ben 10 Radiant War: Swampfire by MisakaLovesYou on DeviantArt

Tormentor (True Omniverse) by PowerMaster17 on DeviantArt

You know it's just occurred to me that the original series had up to 19 aliens in the roster, but if you added Feedback from Omniverse that's an even 20 : r/Ben10

Chromastone And Goop Redesign (Various) by MisakaLovesYou on DeviantArt

Zombozo and Rojo, MHA Style (Rise of Ben 10) by MisakaLovesYou on DeviantArt

Original Ben 10 aliens: Starlequin and Fragon by MisakaLovesYou on DeviantArt

Jen 10 Alien Set: Rainbowgon by EugeneMinev on DeviantArt

Sephiroth / Yog-Sothoth (Ben 10 Radiant War) by MisakaLovesYou on DeviantArt

Ben 10 Ultimate Heatblast Fan Design by MisterMonsterMaker on DeviantArt

Crashhopper and Astrodactyl Redesigns by MisakaLovesYou on DeviantArt

Original Fanmade Ben 10 alien: Bloodbat. by MisakaLovesYou on DeviantArt

You know it's just occurred to me that the original series had up to 19 aliens in the roster, but if you added Feedback from Omniverse that's an even 20 : r/Ben10

Atomix and XLR8 Redesign (Radiant War) by MisakaLovesYou on DeviantArt