Well that all depends on where in the world you live!
If you live somewhere where the pond will not get colder than 20 celcius all winter guarenteed, then yes, your pleco can live perminently in a pond as long as it's got a filter.
If you live somewhere that gets a proper winter like the UK, most of Europe and most of North America then no, if the pond freezes or just gets a little chilled then your pleco will die.
So. where in the world do you live?