I found a couple of these on a tree branch yesterday. They didn't try to scurry away or do much of anything else even though I had cut down the branch, trimmed it, brought it up on the porch, and then took a bunch of photos. Is it a caterpillar? Egg sac? For scale reference this thing about the size of a dime.

Update: thanks to Ted's diligent googling it looks like this is a Spiny Oak Slug Caterpillar.