 Doc mentions that he left Einstein in a suspended animation kennel  in 2015. This line was written in to explain why Einstein was not  with Doc when he came back to 1985 to pick up Marty and Jennifer,  since in _Back to the Future (1985)_ (qv), Einstein had gone with  Doc into the future, but was not there when Doc came back at the end  of the film/beginning of Part II.