The evil inclination gets a definitivedefinite article - יצר הרעהרע, in common usage (in my experience), and in various sources, e.g. בראתי יצר הרע of 'קידושין ל ב.
The good inclination does not get a definitivedefinite article - יצר טוב, in common usage (in my experience), and in various sources, e.g. ותן בנו יצר טוב of the Friday night prayer before Kiddush
And if you'd prefer a Tannaic source, using both in one sentence, see Avos D'Rebbi Nosson 16:
יצר הרע כיצד, אמרו שלוש עשרה שנה גדול יצר הרע מיצר טוב
Why is the Yetzer Harah seemingly a more definitive entity than the Yetzer Tov?