I've put together a set of homebrew monsters and I'm going to go through each one and give a bit a background on each.
The Boneless Man was based on a creature I found in an old Celtic myth somewhere, and to be completely honest, I haven't been able to find it again. I thought it sounded like a fun pest-creature for D&D, so I wrote him up.
In myth, he transforms himself into something a person needs, like a bundle of straw, and then after being stored away he'll magically increase his weight until he is just so heavy the sack or cart or whatever breaks. OK, first let me say, I am happy that I don't live in a world where I would get excited to find a bundle of straw on the side of the road. I left the bundle of straw out of the stats on the Boneless Man I made, because if you told a group of player that a bundle of straw was fortuitously lying on the ground unattended I'm pretty sure they would give you the WTF eyeball, shrug, and keep walking.
OK, so the players see a hooded lantern on the ground. Nobody is around, so whose could it be? "I'll carry it." says the barabrian. Then a mile down the road, the barbarian is huffing and puffing and carry barely carry their backpack. The lantern must be cursed! So, they try to get it out and it won't come out! "Crap!" they exclaim. "Cursed items suuuuuuuck."
The barbarian decides to leave the backpack, except his healing potion is in there and he can't get it out without getting the damn lantern out of the way. So, they decide to carry it to town and see if they can get the cursed removed. Halfway to town the backpack bursts open and a weird little creature rolls around laughing, then teleports away before he can be caught.
"Awww, don't be too made at old Hadly Gob," says the halfling bard in the party, "He doesn't really mean any harm... Hoy! I found a lucky marble!"
So the upside to the Boneless Man encounter is that he will leave a luck charm if there's a halfling in the party because they kind of like halflings. Halflings usually get the joke! It's one-use charm, but still that can save your neck in the right situation.
Here's my half-assed picture of a Boneless Man. Click on it to find the title on DMsGuild and find out more about him and the other strange creatures in Creepy, Crawly & Crazy.