Here's an Easter egg for those who've played and love the first Red Dead Redemption. Bonnie MacFarlane was one of the main characters back then, and she's referenced in RDR 2 in this short scene.
You can find this Easter egg by traveling to the western border of New Hanover. Locate Flatneck Station in The Heartlands and go south from there until you reach the river.
There, you will see a dying, love-sick man who asks you to give a letter to Bonnie MacFarlane.