M: Human-Beasts couldn't resist too...

First was Freya, but already then Arthur subconsciously knew that he need to kill her. (-It. Whatever.)
It was just matter of time when others get Merlin too...
Werewolf was the most harmless, but the most intimidating. At morning he just changed into the shy, stuttering guy, whom Merlin cured (and Arthur scared him half to death).
Vampire was the most sneaky. Disgiused as visiting noble, he had trust and acces, but combo between Merlin's magic, Gaius knowledge and Arthur's wooden stick was invincible - he had no chance.
Mermaid was the most embarassing. That shameless creature bluntly tried to seduce him with his body (upper half at least) and words. So... let's just be glad that Merlin is oblivious and Arthur is fast.
Centaurus was... creepiest. (? is that a word?) His blunt words: "Ride me Emrys..." - cause Merlin to faint and Arthur to go rampage. The most uncomfortable part was that he just hadn't finished his phrase: "...I wish to speek with you". Well, after that, Arthur is banished from sacred forest the home of centaurus.
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Tag to season 2 episode 'The Lady of the Lake'.

Set somewhere after s4/Magic Reveal/bromance(maybe slight hint for Merthur)/humour
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It all starts with Dragons... - [link]
After, there were Snakes... - [link]
Griffin came too... - [link]
And there was the Unicorn... - [link]
-
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Merlin (c) BBC