Behemoth is found in every Kingdom Hearts games. It is a PureBlood Heartless.
 Journal Entries
"Enormous Heartless that appeared in Destiny Islands with Shadows. It led the Shadows in the battle against Sora when he first received the Keyblade. Although Sora defeated it, the island was swallowed by the darkness."
"The Heartless that attacked Sora's home, Destiny Islands. Sora fought valiantly to protect the islands and his memories there."
"This giant Heartless will devour all around it if left unchecked. The dark homing blasts fired from its chest can be turned back on it with a block. Target its hands or, if possible, its head.
TIP: Earth-based attacks like its homing blasts can halve your current HP.."
"A giant, heavy-hitting Heartless that tried to put Sora's lights out. And while Sora triumphed over the Darkside, he wasn't ready for the downside: getting swept into the dark torrent that ensued."
Darkside shares resemblance to Dark Follower but this time, it is a dark blue and not a dark purple. like other Heartless, it has yellow glowing eyes.
- The Darkside is the only Heartless boss to appear in every game.