The Wall of Flesh always drops the Pwnhammer upon its defeat, which can be used to destroy Demon/Crimson Altars(note that bringing the Pwnhammer to a pre-hardmode world will not allow you to destroy Demon Altars there). The first three Demon Altars destroyed will trigger the creation of three new Ores, in the following order:
For every Demon Altar you break, 1-3 Wraiths will spawn immediately. The new ores are created by replacing existing World blocks.
Subsequent Demon Altars destroyed will begin the cycle again, though the fourth to sixth altars will drop half as much ore as the first ones, the seventh to ninth will drop only one third of the ore of the first ones, and so forth. This "multiplier" can be calculated with the formula 1/(ceiling(n/3) + 1 - sgn(n)) where n is the number of altars destroyed. So, destroying the 19th altar would yield one-seventh of Cobalt Ore or Palladium Ore compared to the first altar.
Each time a Demon Altar is destroyed, there is also a chance that a single random tile of rock in the world will be converted into ebonstone/crimstone or pearlstone, which will likely grow to become a new underground Hallow or Corruption/Crimson given enough time.
No matter how many Altars are destroyed, Ore rarity remains set with Cobalt/Palladium being the most common, followed by Mythril/Orichalcum, then Adamantite/Titanium as the most rare.