I am going to try my best at this from memory for metal-only DR bypasses (aka, I will forget a few).Originally Posted by Peter Tamburro
Adamantine:
- Iron Golems
- Flesh Golems
- Stone Golems
- Various Adamantine Defenders
Byeshk:
- Mind Flayers
Cold Iron:
- Bralani
- Ghaele
Mithril:
- Stormreaver
Silver:
- Bearded Devils
- Orthons
- Vampires
Some named monsters of said monster races possess more complex DR, such as the vampire in The Shadow Crypt (Magic, Adamantine, Silver).