I recognize robohash from gravatar mainly.

https://en.gravatar.com/site/implement/images/

Gravatar provides 5 fallback custom avatars:

  identicon: a geometric pattern based on an email hash
  monsterid: a generated 'monster' with different colors, faces, etc
  wavatar: generated faces with differing features and backgrounds
  retro: awesome generated, 8-bit arcade-style pixelated faces
  robohash: a generated robot with different colors, faces, et