When “Outstanding Dog Marron” is sent to the Graveyard during a chain, its effect is activated after the current chain resolves (just like “Witch of the Black Forest”).
When “Mystical Knight of Jackal” attacks “Outstanding Dog Marron” and destroys it, the effects are simultaneous. So “Mystical Knight of Jackal”’s effect is Step 1 of a chain and “Outstanding Dog Marron” is Step 2. “Outstanding Dog Marron” is returned to the Deck and “Mystical Knight of Jackal”’s effect disappears.