Hidden Characters

Yellow Ant
Location: Goldwood
Character: Vela
Requirements: None 

Blue Ant
Location: SS Anubis, Generator Room 2F
Character: Any
Requirements: None 

Red Ant
Location: Ichor, Military Base
Character: Juno, Lupus
Requirements: None 

Green Ant
Location: Spawn Ship, Cargo Sewer
Character: Any
Requirements: None 

Male Tribal
Location: Sekhmet, Channel Area
Character: Any
Requirements: Magenta Key 

Female Tribal
Location: Rith Essa Mine
Character: Vela
Requirements: Mine Key 

Zombie Termite
Location: Tawfret, Treehut
Character: Lupus
Requirements: None 

Beetle Termite
Location: Tawfret, Lost Tomb
Character: Vela
Requirements: None

Metallic Termite
Location: Sekhmet, Spiral Walkway (hover across)
Character: Lupus
Requirements: None 

Purple Termite
Location: Mizar's Palace
Character: Juno
Requirements: None 

Extra Stages

Rith Essa Mine Stage
Location: Walkway (station)
Character: Any
Requirements: Powered-up

Tunnels Stage
Location: Rith Essa, Waterfalls
Character: Vela
Requirements: None

Space Station Stage
Location: Space Station, Basement
Character: Any
Requirements: None

King of the Hill
Location: Cerulean Holding Room
Type: Totem Pole
Character: Any
Requirements: None 

More bonuses:
To get the following multi-player bonuses, you must fulfill the specific requirements during the game.

Cyborg Ant
Location: Various
Character: Floyd
Requirements: Get an 'Expert' rating on all Floyd Missions

Goldwood Target Range
Location: Goldwood
Type: Floyd Mission Award
Character: Floyd
Requirements: Get Gold on Goldwood Floyd Mission 

Rith Essa Target Range
Location: Eschebone
Type: Floyd Mission Award
Character: Floyd
Requirements: Get Gold on Eschebone Floyd Mission 

Jeff & Barry Arcade Racing
Location: Ichor Arcade
Type: Arcade Score Reward
Character: Any
Requirements: Come in first on Jeff & Barry Arcade Racing 

Jeff & Barry Arcade Racing II
Location: Ichor Arcade
Type: Arcade Score Reward
Character: Any
Requirements: Come in first on Jeff & Barry Arcade Racing II 

Greenwood Village Race Track
Location: Ichor Arcade
Type: Arcade Score Reward
Character: Any
Requirements: Break the top records on both Jeff & Barry arcade machines. 

Mizar 3D Racer
Location: Mizar's Palace
Type: Race Award
Character: Any
Requirements: Come in first in the Mizar races.