Dragons,Ungoliant, Ents , Eagles and Huan

[Amarie of the Vanyar]

Just everybody naturally forgets humble fox who saw three hobbits under a tree in the Shire

True [img]smilies/smile.gif[/img] And we have also forgotten the spirit of Gurthang, the River-woman, ...

If Maiar could not reproduce, then where did Luthien come from?

Melian could have children because she choose to incarnate completely as an elf, as is explained by Tolkien in the following quote (bold is mine):

Lúthien was through her mother, Melian, descended also from the Máyar, the people of the Valar, (...) Melian alone of all those spirits assumed a bodily form, not only as a raiment but as a permanent habitation in form and powers like to the bodies of the Elves
The Shibboleth of Fëanor, note 53, HoME 12

Form this text it seems that Tolkien didn't wanted to allow any other Maia than Melian to reproduce, but he may have reconsidered it later and allowed other Maiar to fully incarnate and reproduce themselves ...