I did this drawing a little over a year ago. Here's what I had to say about it then:
I got a couple of guinea pigs recently and as it's October 2015 they could only be named after one special duo. So I decided to draw this and post it on this special day, they day that duo travel to what was once the future to save Marty's kids. This was originally a doodle that hasn't yet been posted on my Working Doodles blog, so look out for that when it appears.