In my mind, I picture the dummy jumping in front of an incoming Lightning Bolt or charging Progenitus to save a fellow battlemate. Poor expendable soul.
Instead of regenerating a creature, preventing all damage to you by one source was my other main contestant. But the effect wasn’t good enough. And preventing all damage for the turn dissolved too much the idea I reflected on the first paragraph.
(mm, now that I think about it, Meat Shield would’ve been a more fitting name…)
For costing I based myself mostly on Herbal Poultice (although with an eye on Pteron Ghost). I costed it a bit more aggressively, since for a mana less in casting and no mana in activation you get a 1/2 body. I never put Herbal Poultice in my Limited decks, but I think I would play Crash Test Dummy.
The 1/2 allows you to get something else besides the ability from it. You can use it to block incoming damage, then sacrifice it to save another blocking creature. With nothing else on the battlefield, and being an early bird at 2 mana, perhaps you can even make it deal some damage through.