To unlock the following, find Crystal Buggy and get it, no matter whatever you unlock Buggy on certain mode.

Buggy	How to unlock
BLZ-808	In Crystal Caverns, Turn Left And Press Button, Then Get Crystal Buggy On Track Attack Exploration Mode.
Carapace	In Beetle Drive, Win Againest Beetle After Passing Rainbow Gate, Then Get Crystal Buggy On Track Attack Exploration Mode.
Hard Corps	In Phantom Dash, Pass Through Last Invisible Wall, Press Switches And Get On Moving Platform, Then Get Crystal Buggy On Track Attack Exploration Mode.
Keffish	In Tumble Down!, Hit First Button On Left, Hit Second Button On Right And Move On Moving Platform, Then Get Crystal Buggy On Track Attack Exploration.
Moot-8	In Hell Hill, Fly Through Tunnel, Then Get Crystal Buggy On Track Attack Exploration Mode.
R-B-MK 4000	Get All Gold Medal On Pub 2, After Entering Pub 3, Then Get Crystal Buggy On Any Mode.
Skratch Thrax XS-2000	In The Trophy Cabinet, After You Unlocking All Other Buggy On All Other Bonus Areas, Then Get Crystal Buggy On Track Attack Exploration.
Soul Shark Buggee	Get All Gold Medal On Pub 3, After Entering Tunnel On Pub 4, Then Get Crystal Buggy On Any Mode.
Stomp!	In Heavenly Highway, Avoid Giant Rainbow Balls And Moving Obstacles, Then Get Crystal Buggy On Track Attack Exploration Mode.
Tasty!	Get All Gold Medal On Pub 1, After Leaving Pub 1, Then Get Crystal Buggy On Any Mode.
Venom	Get All Gold Medal On Pub 3, After Entering Pub 4, Find Invisible Wall And Smash It, Then Get Crystal Buggy On Challenge 2 Mode.
Waspee	In High Helix, Fly Through Floating Platform While Getting Invisible And Pass Through Red Block, Then Get Crystal Buggy On Track Attack Exploration Mode.