Ptmâk is reported in an antehuman text to be the "immediate" parent of Cthulhu (Kthulhut) and his race (the Cosmic Octopi). It appears to have remained dwelling of Vhoorl and is thus quite obscure. Probably another name for Nug.
Behind the Mythos
The letter is written ambiguously about who is a child of Nug: "The immediate parent of Cthulhu and his race (child of Nug) was Ptmâk". Comparing Smith's letter with his and Lovecraft's Family trees of Azathoth suggests that Ptmâk and Cxaxukluth are the Hyperborean versions of Nug and Yeb.