Barrel Spider

Barrel Spiders are enemies from Kingdom Hearts, Kingdom Hearts:Chain of Memories and Kingdom Hearts: Re:Chain of Memories. They are similar looking to the Pot Spider. As noted in the Journal, any contact with fire (Especially Fire, Fira, and Firaga) will make them explode.

Kingdom Hearts
''Gunpowder barrel turned Heartless. They charge into their enemies and explode. Any physical impact or fire will set them off.''

Kingdom Hearts: Chain of Memories
A gunpowder-packed barrel transformed by the darkness into a Heartless.

The Barrel Spider sets upon anyone foolish enough to think it's an ordinary barrel.

Strategy
Because of their explosive properties, it's best to avoid close range battles with them; and instead use Fire, or Blizzard magic on them. However, you can use their explosive properties to your advantage, to stun groups of enemies.

Before showing their true form as a heartless, attacking a suspected barrel with magic can reveal whether or not it is a Barrel Spider. If the barrel blows up, it is not a Barrel Spider, but if it doesn't, it is.

In Kingdom Hearts: Re:Chain of Memories Barrel Spiders pose as regular barrels until they are touched with the Keyblade. The best way to reveal them without going into battle mode is to compare them to other barrels, Barrel Spiders are usually a little darker than regular barrels.