wow that is strange What the heck.. I have never seen anything like that before. I have seen plenty of downed Feros. They often roll down hills in AFNM. Usually they are empty inside..
(except once, one was filled with all those nasty gnats or whatever that eat the flesh after sagauros/feros/opuntia/echinocereus get the flesh necrosis, which yes I have seen it in all of them)
Here's to you, all you insidious creatures of green..er I mean cacti.