Keeping Plecostomus in an outdoor pond in a region with cool temperatures is not advisable due to the risk of exposing them to potentially harmful and even fatal temperature fluctuations. However, if you live in an area where temperatures remain relatively mild and within the appropriate range for Plecostomus, it may be feasible to keep them in an outdoor pond. In such cases, it is crucial to monitor the temperature regularly and take appropriate precautions to ensure the fish's well-being. This could involve providing shelter, such as submerged logs or rocks, and considering supplemental heating during colder periods.
Ultimately, it's essential to prioritize the health and safety of your Plecostomus by keeping them in an environment that meets their temperature requirements. If you're unsure whether your outdoor pond can maintain suitable temperatures for Plecostomus, it may be best to consider keeping them in an indoor aquarium or explore alternative outdoor pond fish species better adapted to cooler climates.