Rex, regem
Rex is a third declension noun. It has a consonant stem and is one of the 'gutterals'. Third declension consonant 'gutteral' nouns decline like so ...
| Case | Singular | Plural |
|---|---|---|
| Nom. | rex | reges |
| Voc. | rex | reges |
| Acc. | regem | reges |
| Gen. | regis | regum |
| Dat. | regi | regibus |
| Abl. | rege | regibus |