If someone needs something like a Fish Needs a Bicycle, they do not need it at all, originally a feminist slogan: A woman needs a man like a fish needs a bicycle.
If people leave a company because they know that it's about to have serious problems, or turn their back on a person about to be in a similar situation, they are said to be like rats deserting a sinking ship.
(USA) If someone or something is lower than a snake's belly in a wagon rut, they are of low moral standing because a snake's belly is low and if the snake is in a wagon rut, it is really low.
The basic characteristics of something is the nature of the beast; often used when there's an aspect of something that cannot be changed or that is unpleasant or difficult.
If you open a can of worms, you do something that will cause a lot of problems and is, on balance, probably going to cause more trouble than it's worth.