Both Galatians 6:15 and 2 Corinthians 5:17 use the expression a new creature or a new creation to describe how vastly different from their former selves and the rest of humanity Christians have become. Most scholars opt for the rendering a new creation but some, such as Donald Guthrie (for 2 Corinthians, but not for Galatians) and Alan Cole (for both passages), prefer a new creature. Herman Ridderbos speaks of a new life-form.
Bibliography
R. Alan Cole The Epistle of Paul to the Galatians: An Introduction and Commentary (Tyndale New Testament Commentaries) The Tyndale Press, London, 1965, page 182
Donald Guthrie Galatians (New Century Bible Commentary) Eerdmans, Grand Rapids, Michigan, 1973, page 151
Herman N. Ridderbos The Epistle of Paul to the Churches of Galatia (The New International Commentary on the New Testament) Eerdmans, Grand Rapids, Michigan, 1953, page 226