When I was young, we had a goldfish pond that we drained every winter and kept the fish in a horse trough in our basement, we took in a pleco from a guy who had it (12") in a 20 gallon tank. Well Mr. Pleco went on to live in our pond during the summer/trough during the winter for about 5 years. Then one day I came home and looked out the back door at the pond, and there stood a Eggret in our pond which I promptly scared off.
After that the pleco was missing, along with a couple 8-10 inch fantail goldfish.
So yeah he could live in a outdoor pond, IF it was warm enough during the summer, and was brought in when it started to get cold out.
Ours did!