It’s been confirmed on multiple post/streams that those are not normal tadpoles and they were created for the needs of BG3. What they actually do, however, remains to be seen. There a bits and pieces here and there but they are vague enough to be easily misinterpreted, and even if accurate it goes into potential spoiler category.
I used to think these were all Ulitharid tadpoles, but I am thinking that I'm mistaken.
