Please mention your own favorite examples. The most notable difference between bosses IRL vs in games is that in games you generally get to beat the absolute tar out of them. No particular order.
T. Rex, Jurassic Park, Nintendo Entertainment System
There are lots of T. Rex in games, but this one eats children.
Gilgamesh, Final Fantasy V, SNES & various ports
Personally love womping on a bootlicker, and G here is a best in show of the breed.
Super Drunk, Bubble Bobble, Arcade & NES
This guy is kind of making fun of the drunks at bars where this game debuted, which is kind of mean, but as a character he is terrible and I relish in his destruction.
Deerhaunter, Timesplitters Future Perfect, Playstation 2 & Game Cube & Xbox
Because he is also playable, "The Moose is Loose" became a common call on game nights in the mid 00s (especially in Ohio).
Decoy Octopus, Metal Gear Solid, Playstation & PC
Controversial because he cannot be beaten without a multi-tap and "foreknowledge" aka you have to actually read the manual where it suggests you might need to read some Cold War history, plug in a multitap and do CQC on him during a cutscene.
That is my recollection as well. Also I remember Psycho Mantis would make some comments about the content of your memory card before the fight.
Another favorite of mine is The End in MGS3: Snake Eater, he is a sniper hiding in the woods and it takes like an hour of game time to beat him down. I seem to recall if you save during the fight and change the date forward a few years on your PS3, when you reload he has died of old age waiting for you to come back.
You can also just ace the dude when he goes out for fresh air before the fight, it's easier than Decoy Octopus but harder than Ocelot in my opinion
edit, clarity: ace the dude in MGS3 sniper battle, he has a generic name i can't remember like that boy or the bad or the line