Answer:
a metaphor. The sentence is comparing John to a snake; they're both sneaky. Another example of a metaphor is:
"Natalia is such a parrot!"
This might relay the idea that Natalie copies everything the people around her does and is a huge trend follower.
One common mistake is mixing up similes with metaphors. Similes include "as" or "like":
"Owen is as sly as a fox" or
"Penny is flexible like a rubber band".
Hope this helped!