Snake

Meaning
Complication, and the person or motive behind it. The Snake is the winding path — a matter that will not go straight, a difficulty that comes from someone else's interests rather than from circumstance. Traditionally it also stands for a woman close to the situation whose involvement is not straightforward, though this reading has been used carelessly enough that it deserves care.
It carries the sense of something coiled: pressure, entanglement, a route that doubles back.
The tone is difficult but not catastrophic. The Snake complicates; it does not end. It is worth separating from the Fox — the Fox is cunning and self-serving, the Snake is convoluted and entangling, and they are not the same problem.
Beside other cards
Usually the modifier, twisting its neighbour into something complicated. Snake + Ring is an entangled agreement. Snake + Letter is a message that is not what it seems. Beside person cards it points to influence from outside.