Giant Bats provide large amounts of health in vertically-scrolling Dungeons, but they can be very dangerous. It's difficult to fight them without touching them, and it's easy to lose track of where you are and be forced out of the Dungeon while doing so. They sometimes spawn Bat Minions as well.
Although "Aqua" refers to their color, you'll find quite a few Aqua Bats in the waters of Warlic's Lake. They can be somewhat dangerous until you have the Valley Sword, but are more of a nuisance than a real threat.
Red Bats are only slightly more dangerous than their Aqua cousins, yet drop four times the gold. They're still not great for farming, since their numbers and attack pattern are bound to get some damage through.
White Bats are the pinnacle of bat evolution, and they are found all over Fosbus' Swamp. They are tough and deal a lot of damage, and are hard to avoid. They drop a nice pile of gold, but hunting them is difficult.