I've seen tapeworms...and it doesn't look like a tapeworm to me. You can tell a tapeworm is a worm-when you slice it, there are tissue structures inside, the outer skin is a little different than the inside.
I'm sure the OP is intelligent enough to figure out that it is a rubber band.
From personal experience, I'd say a guinea worm looks a lot more like a rubber band than a tapeworm does (tapeworms are so flat)....but even then, it also has tissue structures inside of it. Guinea worms are really rare, and need water contaminated with crustaceans (water fleas) to complete their life cycle, so once they're out, they're out. (ya, but lucky for me my college roomate's dog got one and we discovered it after having staggered home from the bar at 3am!!)
If there have been stones, and poop (ew!!!!) and a nickel and a lizard (!?!?!?!) inside an egg....then I guess this is totally plausible. Its crazy cause I suppose it would have had to work its way up the vent somehow to be in the place where the shell is formed. But yk...I've looked at my chooks vents before and they're flexing them...kinda doing 'kegels'...doesn't seem to crazy to think that all the flexing might have moved it upwards.
I think the REAL question is this:
who is bold enough to put on a rubber glove, and get out the ky.... and put something into their hen's vent...something small and inert and sterilized...like a glass marble...to see if it shows up inside the next day's egg??? (for safety, you'd have to keep her in a wire bottomed cage, in case she squeezed it out, so she didn't eat it)
I dare ya all!!!