Amebot (Patreon)

 This is an Amebot, a standard menial labour and combat unit used by Domina. They come into use after she 'convinces' Yellow Diamond to join her, and are converted from gems (and later humans.) The form comes from a mistaken belief that Pearl and Amethyst were in a relationship, and thus an attempt to demoralise the former. 

They're connected to a single hive mind, and when freed, those gems and humans very often have a deep yearning to rejoin the group consensus. This is absolutely under Domina's control and not dangerous at all.