Some monsters are generated with (or pick up) and use shields. Of specific note are Archons, Angels and Aleaxes - all Archons are generated with a shield of reflection, whereas Angels and Aleaxes have a 25% chance of having one. This makes using ray-based attacks against them a Bad Idea.
A humanoid pet with suitable capability will pick up and use shields as well, for example a pet Centaur.
List of shields
|Uruk-hai shield||white-handed shield||7||50||1||iron|
|Orcish shield||red-eyed shield||7||50||1||iron|
|Large shield||large shield||10||100||2||iron|
|Dwarvish roundshield||large round shield||10||100||2||iron|
|Elven shield||blue and green shield||7||40||2||wood||can be enchanted to +7|
|Small shield||small shield||3||30||1||wood|
|Shield of reflection||polished silver shield||50||50||2||silver||provides reflection|
- spell.c#line1216 (extra shield penalty doesn't apply for small shields)