Shambling Mass was the 6th of 9 cards revealed on Twitter and released in Beta 13.4. It didn't change between reveal and release. Previous Versions can be found here.
Shambling Mass has the ability to gain +3 ATK and Haste, but it requires you to kill an ally monster. It will result in giving you a 5G 6/6 Haste, which is quite powerful.
This is obviously best used on Dust monsters in order to trigger their effect. Be warned that it isn't useful as a trigger for Mettaton NEO, as NEO will simply kill off Mass.