DKC Atlas Forum

Confirmed Characters
* Donkey Kong
* Diddy Kong
* Rambi
* Tikis
Enemies
* Mugly (boss)
* Unnamed parrot-like creatures
* Unnamed crab-like creatures
* Unnamed fly trap-like creatures
* Unnamed frog-like creatures
* Unnamed mole-like creatures
* Unnamed shark-like creatures
* Unnamed octopus-like creature
Confirmed Items and Objects
* Bananas
* Banana Bunches
* Banana Coins
* Minecarts
* K-O-N-G Letters
* Barrel Cannons
* DK Barrels
* Auto-Fire Barrels
* Extra Life Balloons
* Bonus Areas
Confirmed Levels
* Jungle Hijinx
* Poppin' Planks
* Rickety Rails
* Mugly's Mound

Source:
http://www.mariowiki.com/Donkey_Kong_Country_Returns

Updated list from Super Mario Wiki:
Confirmed Characters:
*Donkey Kong
*Diddy Kong
*Cranky Kong
*Tiki Tak Tribe
*Pigs
*Super Kong
Animal Buddies:
*Rambi
*Squawks
Confirmed Enemies:
Mugly (boss)
*Unnamed bat-like creatures
*Unnamed moles seen doing various activities such as riding minecarts and throwing bombs at the Kongs.
*Unnamed parrot-like creatures
*Unnamed crab-like creatures
*Unnamed fly trap-like creatures
*Unnamed frog-like creatures
*Unnamed mole-like creatures
*Unnamed shark-like creatures
*Unnamed octopus-like creature
*Unnamed tiki-like creatures
*Unnamed bowling pin shaped gooses
*Unnamed pirate-like crabs
*Unnamed fossilized/mine car-riding dinosaurs
*Wooden Spike Balls
*Unnamed spider-like creatures
*Unnamed giant bat creature
*Unnamed giant bomb-throwing bird
*Unamed leafy-like caterpillar
*Unnamed hippos
*Unnamed whales
Confirmed Objects:
*Bananas
*Banana Bunches
*Banana Coins
*Barrels
*Minecarts
*K-O-N-G Letters
*Barrel Cannons
*DK Barrels
*Hearts
*Puzzle Pieces
*Auto-Fire Barrels
*Red Balloons
*Bonus Areas
*Treasure Chests
*Animal Crates
*Rocket Barrels
Confirmed Levels:
*Jungle Hijinx
*King of Cling
*Tree Top Bop
*Sunset Shore
*Poppin' Planks
*Rickety Rails
*Mugly's Mound
*Crazy Cart
*Button Bash
*Damp Dungeon
*Temple Topple
*Grip and Trip
*Mole Patrol
*Bombs Away
*Muncher Marathon
*Music Madness
*Rocket Launch
*Furious Fire
*Hot Rocket
*Roasting Rails
*Smoking Peak
*Bobbing Basalt
*Moving Melters