The creature is an amphibious creature that lives in and near lakes. It is very large, being about 1.5m high at the thickest part of the body, and 7.5m long in total. Its neck is about 3.5m long and 30cm wide, and is very flexible. The neck is well-defined from the body, but the tail is not. Its head and neck are hard to see when it in the water. Its body is also flexible enough for it to curl its tail around to its neck. It has short, stubby limbs that are hard to notice when it is on land. On land, it moves in a lurching manner. In the water, it moves in a wriggling, churning way, and stays at the surface. Its back also looks like it is made of wood. What could this creature's ancestors be, and how might it evolve in this way?
It is a sauropod.
Your long necked, short-tailed creature is a sauropod. It is an airbreather and so it stays at the surface. It walks on the bottom or punts along with legs off the bottom. It is buoyant and for short stretches can swim like an elephant, using its long neck to breathe instead of the trunk.
A sauropod, this creature is a vegetarian because mostly there are vegetables in the world. The big bodies allow efficient rumen-like digestion. Like hippopotomi, it is not adverse to opportunistic omnivory.
They are used to having the water to support their bulk. On land the big ones usually drag their bellies when they walk in the manner of alligators. That is how they go thru shallow water also. Smaller, lighter ones are more likely to leave the water and use their long necks in the manner of giraffes to forage on trees and vegetation up out of reach of smaller herbivores and away from the water where the big ones get the best plants.
Their backs look like wood because all of their skin is thickly keratinized to protect against the sun and biting flies, and when the backs dry out they look like wood. One that had been out of the water long enough to dry completely would look like wood everywhere.
This creature could have evolved from an early lizard. The reason for the long, flexible neck could be that it is a herbivore: This trait would allow it to eat lots of plants without moving the body. The flexibility of the body would be useful in hiding itself, as it could coil into a ball and hide in a smaller hole. The large size could be useful as an antipredator adaptation, as would camoflaguing the body to resemble a log. At larger sizes, having the legs under the body, rather than out to the sides, would be helpful. This could also conceal the legs. The amphibious nature may allow them to find more food. Being on the surface wouldn't be much of a detriment, as the long neck would allow it to reach plants deep in the water without diving. Finally, the lurching, churning motion could be explained as caused by symmetrically moving flippers out of sight, either below the water or under the body
It needs Food. Lots of food.
Plus a few other conditions.
If it is a herbivore, it needs a food source that must be reached using a long neck (else, why have it). It needs predators that are defended against by being big. It needs either a very low population so there is likely to be food available when it moves because it ate everything around it. Then it needs to be able to find a mate. Low population means that a mate is not close. They could be herd creatures which means that a mate is easier to find but that they would have to move more often.
If it is a predator, it needs to have enough prey to support itself. With the long neck, it could be an ambush predator. However, the prey would have to be very plentiful and it would have to not be able to learn to avoid that area. If the prey is migratory, your creature should be able to hibernate between feasting or have a very slow metabolism. Floating its main body while just using its neck for hunting would save energy.
What you're describing sounds more like a plesiosaur, but if it must be amphibious, it could be a large long-necked amphiuma or siren, which are giant aquatic salamanders with small stubby legs. Perhaps it eats mammals and birds at the edge of a water like a crocodile, but to save energy it evolved a longer neck so it doesn't have to move as much when grabbing its prey from the bank.