It's about Morphing Jar #2. I was curious, if I for example had special summoned like a Lava Golem to your opponent's side of the field, and then Morphing Jar #2 is flipped. What would the outcome be? Lava Golem is on their side of the field. Yet; it is returning to the original owner's deck(mine). Thus; I'm clueless. Help please. :)

Morphing Jar #2
Earth/Rock/3/800/700
FLIP: Return all Monster Cards on the field to their respective Decks and shuffle them. You and your opponent then pick up cards until you both have the same number of Monster Cards that were returned to each Deck. Special Summon the Level 4 or lower monsters to the field in face-down Defense Position. The rest of the cards picked up are discarded to the Graveyard.

Unless an effect specifically says otherwise, Lava Golem goes back to your own Deck and will be counted towards the number of Monsters YOU go through while picking up cards.

So if you had 2 monsters on your field (excluding Morphing Jar #2 since I'll just say it was attacked and killed in battle) and the opponent had your Lava Golem on their field, then you had 3 monsters returned to your Deck (your 2 and Lava Golem) and so you would pick up monsters until you go through 3 monsters.

So you pick up cards and, let's say, picked up Slate Warrior, Harpie Lady 1 and Roc from the Valley of Haze. Right after Roc, you would stop picking up cards since you picked up the same number of monsters that were returned to the Deck. (3 monsters) Slate Warrior and Harpie Lady 1, when picked up, would be Special Summoned to your field in face-down Defense Position and Roc would be sent to the Graveyard. (note that Roc would NOT return to your Deck since it is being sent from the Deck to the Graveyard, not the hand)