Let's do a little math here.
Based on my research, the voltage to produce a half-meter long spark in dry air between two needle-shaped electrodes is roughly 300,000 volts. This would indicate that the critter would need to have some sort of modified fang or tusk in its mouth to concentrate the charge. A quick search of the web puts the electric eel around 600V, with unconfirmed reports of over 800V, however, that's directly tied to the size and number of the eel's generating organs, so we're gonna need a bigger critter, possibly 500 times bigger. A typical electric eel is about 20kg, putting a nice ballpark on size. We're looking at 10,000kg of critter here, about half again the size of a bull elephant.
An electric eel can generate roughly one amp. Typical TASERs operate somewhere in the ballpark of 26 watts. Given that we're dealing with 300,000V, that means we only need roughly .09mA to incapacitate a grown man. That seems easily in reach for our school-bus-sized thunder-monster. If it could still produce the full amp that an eel could, even just for an instant, its shock would be horrifyingly lethal.
Now, the critter would have to insulate itself from the ground to avoid that lightning arcing through their body from their mouth to their feet. One organic product comes to mind that is a superb insulator: beeswax. The critter would probably either excrete the wax from pores in the skin, or spend a lot of time grooming itself with a waxy gland. I imagine that this would also give the creature a nice dull, dry gloss. Its leather would probably also make a good pair of naturally waterproof boots.
That, all by itself, would be enough to get you your lightning-breathing monster, but one other improvement comes to mind: mucous filled spittle. Spit is a fine conductor, and with enough phlegm, you can get some pretty long strings going (source: my teething son), which could improve the range of the sparks nicely.
To sum it all up, I would imagine a twenty-ton, hairless beast with a thick coat of gleaming wax covering its body. As it opens its mouth, a single wicked-looking fang juts out. Its massive bellow shoots out streamers of lighting and noxious spittle. Anything standing in its way has mere milliseconds left to live before they fall dead or burst into flames.Bursting into flames may or may not be an exaggeration, but terrible scorching would definitely happen.
Finally, as to how it would evolve, I can easily imagine a smaller ancestor biting its prey before releasing a more modest shock that would leave its victim stunned and helpless.
