Unless there is a viable cold fusion process that we don't know about, the energy, pressure and temperatures required for nuclear fusion are not attainable in any "evolved" way. We can barely manage to "intelligently design" a viable fusion reactor with all our technology. The odds that all the intermediate steps would prove beneficial enough to be selected for are unimaginably low.
Spending a week's worth of energy to raise internal temperature to 100 degrees might have some remote benefit. But a creature that burns off a year in feeding to attain 10,000 degrees for a few minutes, without any actual benefit, is going to get outcompeted by creatures spending that energy on growth, claws, brains or other useful traits.
If it did exist, it would probably be a Dyson Jellyfish. This creature grows in space, orbiting a young star where planets have not formed yet. It lives on solar power and absorbs all the dust and gas in the accretion disc, until it can surround the star entirely. At that point it grows inward to manipulate the star with extremely strong magnetic fields, allowing it to trigger solar flares at will.
It will reproduce by budding, slingshotting the buds past the sun and out through holes it creates in its shell. In a million years or so, they reach their target gas cloud, by now a star, and the process repeats. This couldn't have evolved naturally in the life time of our universe, I'm afraid.