so and so = فُلان

فُلان (fulan) without an ال attached to it means so-and-so, or such a person, i.e. as a substitute for an unnamed and unspecified person. It is a triptote noun, but its feminine فُلانَة (fulanah) is a diptote; e.g. قَالَ إِنَّهُ رَأَى فُلانًا وفُلانَة he said he saw such-and-such a man and such-and-such a woman. When we want to use it for non-rational beings, we attach an ال to it, e.g. رَكِبْتُ الْفُلان I rode such-and-such an animal, and حَلَبْتُ الْفُلانَة I milked such-and-such an animal. It is useful when the exact name or identity of the person or thing being referred to is not know, or has been forgotten, much as in informal English we might say, I went with what’s-his-name, or he said he saw that guy there.