"How do I make a Fear?"Write a story! Either plan the monster out in advance, or establish the theme of your story and label the Fear afterwards that way! ..or really do it any way you want. But how about, for the sake of arbitrary convenience, we assume here that "Fear" means a horror monster? It'll make the following points easier to grasp.
"Is it still a Fear if it hasn't appeared in a blog/vlog/creepypasta/anything yet?"No. Then it's just an idea and you can't claim ownership of it so easily.
(AND THAT BRINGS US TO)
"Speaking of ownership, if I make a Fear, does that mean other people can automatically write with it?"Well. Yes, technically. But it's a little more abstract than that. They can't literally write with the exact Fear in your story-- they can't write with your portrayal, and that is what matters here. For instance, I can't stop people from writing with the Fear of Depersonalization and calling it "EAT" and making it a hive mind of water-y stuff. Because that's just a bunch of ideas. I can't even stop people from calling it "Salmacis" and making it focus on blue-haired bodies! Again, those are literally just ideas. But I can stop them from writing in my actual canon, with my actual specific portrayals. They cannot change my stories' Fears without my permission. But I cannot stop them from writing their own stories, even if their Fears are identical to mine. Because their stories are, by default, in their own canon. They do not affect my stories whatsoever. They would get their own separate Wiki articles (we haven't done that much in the past but dammit we will start!), and they could not affect my actual stories and my actual Fears without my permission.
And that is all that matters. If you don't want people to be able to mess with your Fear, you will fit right in. (Even if you do, you're always welcome to specifically state your canon is open-source, which most of our canons aren't, or you can ask to collaborate with people! That is always fun! :D)