In Latin the similarity between the nominative plural and genitive singular is most striking:
- First: porta (Nom/Sing) and portae (Nom/Pl), portae (Gen/Sing) and portarum (Gen/Pl)
- Second: servus (Nom/Sing) and servī (Nom/Pl), servī (Gen/Sing) and servōrum (Gen/Pl)
- Third: māter (Nom/Sing) and mātrēs (Nom/Pl), mātris (Gen/Sing) and mātrum (Gen/Pl)
- Fourth: manus (Nom/Sing) and manūs (Nom/Pl), manūs (Gen/Sing) and manuum (Gen/Pl)
- Fifth: fidēs (Nom/Sing) and fidēs (Nom/Pl), fideī (Gen/Sing) and fidērum (Gen/Pl)
In the First, Second and Fourth declensions, Nominitive Plural is the same as Genitive Singular. The other two don't follow the same pattern, but don't deviate very far from it.
Even in English, we say:
- dog (Nom/Sing) and dogs (Nom/Pl), dog's (Gen/Sing) and dogs' (Gen/Pl)
There are some other examples I've found especially while studying Russian:
- ко́мната (Nom/Sing) and ко́мнаты (Nom/Pl), ко́мнаты (Gen/Sing) and ко́мнат (Gen/Pl)