I so often hear people talking about how easy and cheap a certain pet is to take care of, and very often that is not the case.
In my book, there really isn't such a thing as an easy pet. Some pets are more difficult to care for than others, but taking care of any pet requires knowledge, time, money, work, and commitment.
Here are some pets labeled as "easy" pets:
Fish- fish may be easy for someone who has a lot of experience with them, but for beginners fish keeping is a whole new world.
For me I find that Squirt is easy to feed, and cleaning his tank is a breeze, but with fish there are no fish vets. That is up to the owner. Diagnosing and treating fish ailments is anything but easy.
Rabbits- It is said that rabbits are easy pets to care for all the time. Well, I have never really met a rabbit that is easy to take care of. Rabbits require a lot of attention and companionship. They require daily cage cleanings and regular feedings of hay, pellets, and veggies, and they require a lot of space. They are also very mischievous and love to get into trouble and chew everything that they shouldn't, so they need a lot of attention and toys to keep them from being bored. And they are definitely not cheap to take care of (properly that is).
Guinea pigs- They may be fairly easy to handle and pet, but that is where easy stops. I LOVE guinea pigs but they are MESSY. They need a spot cleaning every day (sometimes more than once a day) and they need a big top to bottom cage cleaning once to twice a week. Their diet requires a lot: pellets, hay, veggies, and you have to be sure they are getting enough vitamin C. Hay and bedding for guinea pigs can be pretty pricey.
Hamsters and mice- Granted, they are easier to take care of than a lot of pets, but if you want a really good cage, complete with the tubes and such, cleaning it is going to take a while. (Hence most hamster owners, including me, only wash the tubes when they actually need it. Everything else I wash once a week.)
If they live in a tank, cleaning can be a bit easier, except that the tank is heavy.
Reptiles: Caring for reptiles can be easy, but only if you know exactly what you are doing. A lot of knowledge is required for reptiles. Owning a reptile also means you will have to be willing to take care of crickets, worms, and even mice and rats to feed them.
It is funny, I never hear anyone say that gerbils are easy to care for, and in my experience they actually are a lot easier to care for than most pets. As long as they have a spacious tank and LOTS of (safe) things to chew on (along with a water bottle, bedding, a hideout, and a wheel), all they require is a cage cleaning every two weeks, and daily feedings. Those few things plus some time in a ball, and interaction with their human(s), and that is about it!
Although... they are definitely not the easiest to handle. They are fast! But they generally don't bite.
The only TRULY 100% easy pet to care for is.....
a pet rock.