Purple Tentacle

Humble Beginnings

Originally a creation of Dr. Fred Edison in Maniac Mansion, Purple Tentacle served as a minor antagonist that slowed your progress through the latter part of the game.  He's hated by Green Tentacle, but beyond that you don't learn much.
 

In Day of the Tentacle, however, a new side to your former nemesis reveals itself.

With little to distinguish him from fellow tentacles other than a menacing unibrow and his eponymous hue, Purple Tentacle began earnestly enough ambling through the less than idyllic countryside behind Dr. Fred Edison's home.  In the opening scenes of Day of the Tentacle, however, Purple Tentacle was thrust into the world of world domination faster than he could grow a pair of frightening apendages (arms) and quickly come to terms with his newfound goal in life, "to take on the world."
 
He is the primary foe in this adventure game sequel, beginning with serial cow tipping and only becoming MORE diabolical from there.
 

The game, which takes place five years after Manic Mansion, opens with Purple Tentacle becoming exposed to toxic waste spewing from Dr. Fred Edison's mansion. Purple's ingestion of it results in him growing a pair of flipper-like arms, vastly increased intelligence, and acquiring a thirst for global domination. Dr. Fred catches Purple Tentacle, as well as the friendly, non-evil Green Tentacle, and keeps them both in his basement before deciding that he will euthanize them. Bernard frees Green and Purple, only for Purple to inform him of his plans of world domination and resume his conquering of the Earth. Since he can't figure out a way to stop Purple now that he's begun his mad quest, Dr. Edison attempts to send the three friends back in time to 'yesterday' using his time Machine. By doing so, they can turn off the sludge machine which produced the toxic waste so that Purple never ingests the waste in the first place, hence pre-emptively stopping him from taking over the world. 

Upon arrival, they find that the Purple Tentacle from 200 years in the future has also used the time machine to bring several versions of himself to the same day to prevent them from turning off the sludge machine. Bernard and his friends successfully defeat all the Purple Tentacles, turn off the machine and restore the course of future events to normal order.

Side Jobs

 
While not an adventure game, Zombies Ate My Neighbors featured Tentacle as one of the enemies you face.
 
Quotes:
 "I feel like I could... like I could... like I could... TAKE ON THE WORLD!!"   
 
''Humans are our oppressors! They made us live in this horrible motel! They created us in ungainly forms so we could not rise against them! Try walking around with your legs tied together and glue on your shoes, you'll see what I mean! ''    
 
 ''Yes. Thank you, naive human. Now, I can finish taking over the world! ''