Here's my theory: a producer, a director, and a big-name star are having drinks. Somehow the subject of the Loch Ness Monster comes up, everybody talks about how there hasn't been a good Loch Ness Monster movie in years, and, you know, maybe we should have a meeting about that or see if there are already any good Loch Ness Monster scripts floating around. Everybody goes home thinking about what kind of Loch Ness Monster movie they want to make.
Maybe there's some early talk about collaborating on the project, but, in the end, everybody has their own ideas and heads off to develop their own Loch Ness projects.
And, three years later, audiences are scratching their heads and wondering why there are suddenly two Loch Ness Monster movies coming out in the same year . . . .
(The third one got stuck in Development Hell, or lost Nessie in the fourth draft and is now about vampire mermaids.)
That makes a lot of sense. They're not intentionally copying each other, they're just exposed to similar memes because they're part of the same culture -- both in terms of the general public's culture and the specific culture of the film/TV industry. Nothing exists in a vacuum, and nothing has just one influence.