According to this article, all of the H.P. Lovecraft's writings which were published during his lifetime are now in the public domain. As such, there should be no legal/copyright issues arising from your use of that material.
Whether derivative works are guilty of plagiarism, is a bigger question and the internet is full of carefully worded, non-legally-binding (i.e. IMHO) articles on the subject. Here is a good example of one of those.
I would personally build upon the "proper citation" aspect of fair use, if I wanted to reference a Cthulhu creature in something I wrote. I would make sure that the reader knows where the idea for the creature came from. In a fictional setting, how I handled it would be based on the answer to the following question?
Did H.P. Lovecraft live and publish books in my fictional universe?
If he did, then I would have one of my characters be an H.P. Lovecraft fan and have him exclaim upon first encounter with an Old God (or more survivably, an Old God's minion), "Hey, that reminds me of something from one of my favorite books by H.P. Lovecraft!"
If Lovecraft did not exist in my fictional universe, then since I was borrowing one of his creations, I would borrow all of his creations. That means that every character and event from his writing would somehow be a true event (or psychotic person's raving) in my universe's history. Now, a journal by Francis Wayland Thurston, (the narrator of The Call of Cthulhu) would find its way into one of my character's hands. In this case, my making an honest attempt to give proper attribution will hopefully be enough to keep the plagiarism police, flamers and critics away from my door. (Sort of like leaving an I.O.U. in place of the $20 you stole out of Dad's wallet. It may still be stealing, but the chance of getting punished for it is greatly reduced.)
The last point I would offer on this subject is the need to make your work additive to the whole of Lovecraft's world. Don't just borrow his characters and creatures for an uneventful stroll. That is worse than plagiarism. It's boring! Do something creative with these borrowed creations. Reveal something blood chilling about their eldritch nature, or about humanity's origins in relation to the Old Gods, or about how our new technology stacks up against their old lore.
Don't just write another H.P. Lovecraft story. Build upon Lovecraft's world!