The claim of a 50-meter-long mermaid discovered in an abandoned house, supposedly millions of years old, falls more into the realm of urban legend or viral folklore than documented science. Archaeological and scientific standards generally find no record of such a creature, as no fossil evidence supports the existence of mermaid-like beings in prehistoric history.

If such a discovery were real, it would be unprecedented in the field of paleontology and likely studied rigorously to understand its origins. Ancient myths and folklore from cultures worldwide frequently feature mermaids or similar sea creatures, but these stories are generally understood as products of human imagination, inspired perhaps by sightings of known marine animals like manatees, which early sailors mistook for mythical creatures.

Fossils of prehistoric marine life do exist, including massive marine reptiles like the ichthyosaurs and plesiosaurs, which are often misinterpreted as evidence of legendary creatures.

In summary, the idea of a 50-meter-long mermaid is scientifically implausible, as no vertebrate on record reached such a size without being recognized by modern paleontology. This legend would make for an intriguing tale or science-fiction scenario, but it has no foundation in documented history or evidence in known fossil records.