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.