AV1 has source code from VP10 (never published) of the VP8/VP9 fame. It is essentially what VP10 was going to be plus Daala and Thor.
https://en.wikipedia.org/wiki/Daala (Xiph/Mozilla)
https://en.wikipedia.org/wiki/Thor_(video_codec) (Cisco)
https://en.wikipedia.org/wiki/VP9#Successor:_from_VP10_to_AV... (Google)
Updated: to clarify that AV1 is not only VP10 but also Daala and Thor, credit to iskin.