With a dictionary, I can
Nom. sing. oppidum Rōmānum, pl. oppida Rōmāna
Acc. = Nom.
Gen. sing. oppidī Rōmānī, pl. oppidōrum Rōmānōrum
Dat. sing. oppidō Rōmānō, pl. oppidīs Rōmānīs
Abl. = Dat.
Voc. = Nom.
I don’t know if there’s an established Latin word for werewolf, but we can coin lūnavir from lūna (moon), vir (man) and give it the declension of vir. A female werewolf would be called a lūnavitrīx and use the third declension like other -trīx nouns.
So Lūnavir est in oppidum! (There is a werewolf in the town!)
Or we could change the stress: Lūnavir in oppidum est! (There is a werewolf in the town!)
Cool thing: the were in werewolf is thought to share a deep-rooted origin with Latin vir (v was pronounced as w in Classical Latin): both meant “man”. eg. weregild was a fine for killing a man.