There are enormous world eaters that consume planets and digest every atom that goes inside them. You can use anything that is possible with science/technology, but no magic. The world eater does not have to receive 100% of the energy in the consumed matter, all that is important is that every little bit of matter is used in some way.
The process does not have to be quick on a geological timescale. However, all life should cease on eaten worlds within days if not instantly.
I would prefer not to use anti-matter, but the correct answer will contain another process that releases energy from nearly all elements on the (naturally occurring) periodic table as well as how that energy can be harvested.
I know the title says animal, but a space faring animal that eats planets might end up looking a bit like a machine, and that's okay.
The world eaters consume planets, stars, even nebulae for energy to move their enormous mass and to grow larger and consume even larger structures.