Believe it or not, it's a bag of dog poop. That's the sort of thing you leave on someone's front porch, set on fire, ring the doorbell and run like hell.
While riding trails in Phoenix and surrounding area, it is fairly common to find these bags on the side of trails. They are usually green, but occasionally black, like this one. Here's the deal. The law here requires that trail users who bring their pets are required to remove the their pet's droppings. The parks, HOA's and cities place pedestals at trail heads with free bags and a bin to dispose of the bags once they are used, along with instructions for use and disposal.
Occasionally, you will see a dog walker with an empty bag hanging out of one of his or her pockets. I doubt I have ever seen a dog walker with a bag of dog poop on the way back to the trail head. More frequently the bags are left on the side of the trail. The bags are biodegradable, but something tells me they actually slow the dog poop degradation process itself. I often wonder what goes through the mind of the dog walker who leaves the bag on the trail;
"I'll pick it up on the way back."
"Ooh, that's gross! I don't want to touch it!"
"Somebody else will pick it up. That's why I pay taxes."
"I'll send my Valet to pick it up later."
"It's not my dog."
Just one statement for the dog walkers out there, Pick up your dog's crap. You're the one who fed it!