The description of the sorcerer subclass "Shadow Magic" (XGtE, p.50) begins with the following paragraph (the emphasis is mine):
You are a creature of the shadow, because your innate magic comes from Shadowfell itself. You can trace your lineage to an entity from that place, or maybe you were exposed to its fallen energy and transformed by it.
From the top of my head, I can only think of Shadar-Kai (MToF, page 62). Apart from that, I can not think of suitable creatures: Darkmantles or Skull Lords among which are not even humanoids, and it is difficult to imagine Meazels (MToF, p.214) or Skulks (MToF, p.227) having any offspring.
Is there any entity in Shadowfell known from 5e or earlier versions, apart from Shadar-Kai, which is known to reproduce with other humanoids?
Or does "entity" include an immaculate conception of the Raven Queen? :RE