If you Tribute Summon “Thestalos the Firestorm Monarch” successfully, and your opponent discards a monster like “Outstanding Dog Marron” or “Regenerating Mummy”, you still inflict damage because the monster was discarded to the Graveyard (even though it doesn’t remain there).
If “Banisher of the Light” is face-up on the field, no damage is inflicted because the card is not discarded to the Graveyard.
GoatRulings.com:
If “Thestalos the Firestorm Monarch” is tribute summoned using “Sangan”, “Sangan” will be CL1 and Thestalos will be CL2. This is true even if the “Sangan” that was used as a tribute was the opponent’s through the effect of “Brain Control”, “Soul Exchange”, etc.