Upon founding Slytherin House, Salazar Slytherin was adamant on selecting students who possessed characteristics he most valued: one who is ambitious, resourceful, and cunning.
The animal emblem of Slytherin is the snake, with green and silver, and water being the colour and element respectively.
This heralds back to the serpents place in mythology, especially in Western Europe where there are lochs and various other bodies of water. Both water and snakes are fluid and flexible.
Looking closely at Slytherin characteristics, you could go big with your design, with a half or full sleeve, similar to that of traditional Japanese tattooing. Or, if you’re wanting something a little smaller, use these tattoos as inspiration; get your creative juices flowing, and brainstorm the kind of imagery that you want to see in your own Slytherin tattoo
It all comes down to whether you want the piece to be a literal or more obscure reference to Harry Potter.