Final answer:
Ned's error in perception is likely due to misinterpretation or misidentification, influenced by cognitive bias and the altered appearance of the carcass due to decomposition.
Step-by-step explanation:
Ned likely made an error in perception due to a phenomenon known as misinterpretation or misidentification. When encountering an unrecognizable entity, especially in an unexpected context like finding a carcass on a beach, people might jump to conclusions that align with extraordinary explanations such as a 'sea monster'. This form of cognitive bias can be influenced by various factors such as the rarity of the object, the individual's belief system, expectations, and the influence of cultural or media depictions of mythical creatures.
Decomposition may also play a role in the misidentification, as the natural process can alter the appearance of a carcass significantly, further leading to errors in perception. Especially in biology, it's crucial for observations to be grounded in empirical evidence rather than assumptions.