User blog comment:Gourgeist/Who Is The Beast Titan?/@comment-59.189.71.132-20151113105504

It is Erwin Smith's Father, https://i.imgur.com/Cd381Op.jpg, this picture shows his left arm being burned by the government guards.

The Official wiki picture of the beast titan's human form is this, http://vignette1.wikia.nocookie.net/shingekinokyojin/images/f/f7/Beast_Titan_human_form.png/revision/latest?cb=20150616065803, with his left arm having a scar. This is undeniable evidence.