A list of ALL the bosses and their music tracks! If you know any more songs of bosses feel free to reply and I'll edit them in!
((THIS LIST IS EXTREMELY INCOMPLETE BOTH WITH BOSSES AND MUSIC))

Abe - Tribute to Abe by Just Neo | http://youtu.be/V_AGrMu6pD4
Administrator - Administrator Theme Song by ??? | http://youtu.be/JyFJq30Kjsc
Alien - Gimme All Your Lovin' by ZZ Top | http://youtu.be/k4VFFBCa5Aw
Alien Queen - ???
Applejack - ???
Ass Pancakes - ???
Aya Syameimaru - Mysterious Mountain from the game Mountain of Faith | http://youtu.be/sPTOQd9E6gE
Big Daddy - ???
Bill from L4D - The Veteran Guy by Mastgrr | http://youtu.be/Ekl0evT4guM
Bugs Bunny - ???
Captain Kinky - SexyBack (Instrumental) by Justin Timberlake | http://youtu.be/zg9EU3bCyl0
Christian Brutal Sniper - The Millionaire's Holiday by Combustible Edison | http://youtu.be/0VMoIv0oWdc
Cuddly Heavy - Epic Sax Guy (Moldava Eurovision 2010) | http://youtu.be/gy1B3agGNxw
Demon Chef - Disco Inferno by The Trammps | http://youtu.be/A_sY2rjxq6M
Demopan - No music | http://youtu.be/g4mHPeMGTJM
Freddy Krueger - Krueger (Nightmare on Elm Street Theme Remix) by MMSYSTEM | http://youtu.be/deg3-09wLec
Gabe Newell - ???
Garfield - Wooly Bully by Sam the Sham and the Pharaos (??? remix) | ???
Gentle Spy - The Blue Wrath by I Monster | http://youtu.be/R3nts7RZXdQ
Grinch - ???
Heavy Sauce - Barbra Streisand by Duck Sauce (Heavy Sauce remix) | http://youtu.be/ZiACibFEMkQ
Herobrine - Cat by C418 (Caution & Crisis remix) | http://youtu.be/HxHrY37psIU
Homer Simpson - No music | http://youtu.be/g4mHPeMGJTM
Luigi - Bowser's Inside Story Boss Theme Remix by EmeraldoTheDragon | https://youtu.be/m8Q0plaLEfs
MeeM - ???
Merasmus - No music | http://youtu.be/g4mHPeMGTJM
Munch Oddysee - Munch's Theme Remix by bekikuu | http://youtu.be/DV6g4NoDLjg
Ninja Spy - ???
Pac-man - Pac-man (Power Pill remix) by Aphex Twin | http://youtu.be/kjEDX-2M7Qk
Pedobear - No music | http://youtu.be/g4mHPeMGTJM
Pink Panther - ???
Polish Nurse - Machine Gun by The Commodores | http://youtu.be/2iMg3CC6-RE
Pyrogas - ???
R2D2 - ???
Rappy 'n Demoman - Puppy by Netsky | http://youtu.be/FU4cnelEdi4
Robbie the Rabbit - ???
Sandman - Mr. Sandman by The Chordettes (Jan Casre Remix) | http://youtu.be/SdsuwsA7mdM
Saxton Hale - ???
Seeman & Seeldier - No music | http://youtu.be/g4mHPeMGTJM
Skelescout - The Massacre (Version 2) by FantomenK | http://snd.sc/1a3SvMT
Space Marine - Back in Black by AC/DC | http://youtu.be/pAgnJDJN4VA
Sonic - ???
Super Meat Boy - Meat Spin (Spoiled R0TT3n remix) | http://youtu.be/u5FWQCvYTU0
Triceratops - ???
Wall-E - No music | http://youtu.be/g4mHPeMGTJM
Wheatley - ???

Hi can i help you ?

Alien : http://www.youtube.com/watch?v=k4VFFBCa5Aw
Space Marine : Back in Black http://www.youtube.com/watch?v=pAgnJDJN4VA
Bill from Left4ded : http://www.youtube.com/watch?v=Ekl0evT4guM

( I'll edit my post if i find moar )
Have a nice day

Thank you, I've added them in! Please just make a reply to the thread if you find more else I won't see you did

New songs incoming !

Christian Brutal Sniper : The Millionaire's Holiday http://www.youtube.com/watch?v=0VMoIv0oWdc

Freddy Krueger : http://www.youtube.com/watch?v=deg3-09wLec

The administrator : http://www.youtube.com/watch?v=JyFJq30Kjsc

You forget Ninja Spy shame on you !

And Gentlespy ! http://www.youtube.com/watch?v=R3nts7RZXdQ

And Heavy Sauce : http://www.youtube.com/watch?v=ZiACibFEMkQ

Herobrine : https://www.youtube.com/watch?v=DZx9gApWLZY

Luigi https://www.youtube.com/watch?v=m8Q0plaLEfs

I'm tired i'll write the others tommorow !

Thanks I already added Herobrine in, the rest is in now ^-^

The theme I put for Herobrine is C418 - Cat (Caution & Crisis Remix) https://www.youtube.com/watch?v=5hiyE_V5xJA

Skelescout's theme is actually a slightly different version.
It's 'The Massacre (Version 2)' by FantomenK

Pac-man is Aphex Twin - Pac-man (Power Pill remix) http://www.youtube.com/watch?v=kjEDX-2M7Qk

Polish Nurse - Machine Gun - The Commodores (1974) http://www.youtube.com/watch?v=2iMg3CC6-RE

All added in ^-^

Rappy n' Demoman - Puppy - Netsky http://www.youtube.com/watch?v=FU4cnelEdi4