Depends on what you specifically want.
You want a "Poison Mage"
Poison - can either refer to just a DoT as a mechanic (which can be thematically different, but mechanically the same. For example a DoT Champion would be Brand or Malzahar, where a poison Champion is more limited), or you can refer to Poison as a theme, in which case Teemo and Cass are viable responses. Even if they deal damage through other mechanics, their theme is still tied to poison. Teemo doesn't actually kill you by annoying you. He does, in fact, kill you via poison damage. EXCLUSIVELY. He passively deals poison damage on hit. His Q blinds you (which thematically is a "blinding dart" which I see as a form of poison/toxin) and his Shrooms poison you. Literally the ONLY way he can kill you is via some kind of poison-like effect.
Mage - Again, do you just mean "magic damage" or do you mean "spell casting, book-wielding magician"? This also causes problems. Thematically when we think of poison its usually organic (hence mushrooms and toxic clouds), not something arcane like Ryze's abilities.
So if you want, for example, a Champion that uses Poison [Theme] but does not have to be a Mage [Theme], then you could say that
is a good "Poison-based Champion". He passively poisons his enemies and often gets kills by stacking it up and expunging them. Literally killing you via poison.
But if you specifically Poison "Mage" I don't think you'll get much closer to Teemo.