Are they ... artificial cows? Some sentient variety of those mechanical bulls rodeo bar patrons so enjoy hurting themselves with?
What you're looking at are semi-autonomous creatures known as Legged Squad Support Systems, or LS3s. A robot technician walks in front of them and gives them commands, which they seem perfectly content to obey ... for now.
An LS3, seen here nonchalantly blocking your only way to safety.
Also known as DARPA Mules, these weird headless horse creatures are meant to help soldiers haul their gear around the battlefield, although no one will ever convince me the Army isn't secretly planning to ride these things into battle. An LS3 has a carrying capacity of 400 pounds, or two or three humans it has hunted down -- a task it will find extremely easy, thanks to its advanced voice recognition technology. Yes, LS3 knows how to locate you by sound.
If you think that LS3 looks less threatening and more kind of clumsy and harmless, it's only because you haven't seen it in action. Cut to 0:33 of this innocent demonstration video: The guy is walking alone in the forest when suddenly an LS3 enters the frame from the right, stalking him silently save for the quiet revving of its engine and the monotonous thud of its robot murder hooves.