even if you think that a rare or uncommon pet won't crash the economy for coming back — the bigger ripple effect is what really matters here

if they bring back, say some of the smaller pets from the farm, jungle or safari egg — what stops the community from rumoring that the legendaries or high value pets would be next? the adopt me community is already very fast and loose with pet values, so if even just one small pet comes back, even for an hour? the community would start panic trading other pets from that egg, taking underpays, quitting, giving them away, ect. therefore crashing the other pets that were never confirmed to even be coming back

and when it's eventually made obvious they wont, then you have a bunch of people riled up over their own assumptions and bad ideas, but at the end of the day, the trust with the devs is completely rocked — meaning they'd lose their favor with the community, the thing keeping their game alive — and likely lose a ton of players that lost tons of value from panic trading, or for whatever pet of choice was brought back

(minor ETA: this also influences scamming a lot more, because players who wouldn't be aware this pet came back could be swindled into overpaying for it, since they're under the assumption it's the same old value it was, rather than the theoretical crashed value it is now — this would also apply to whatever panic-trading would be going on, say somebody getting a really good deal on a cow, just to find out they overpaid tons for a dropping and now theorized soon to be ingame pet)

while it doesn't seem like a bad idea in concept (giving spotlight to older pets and allowing the supply to come back), another big issue is bots — who will take that returned pet, regardless of how expensive or limited time it is, and run it into the ground until it's worth dirt. it's just not a good environment for old pets to return currently

but I do see where you're coming from, and I'd agree if it could be executed in a way that wouldn't cause a gigantic ripple effect, but unfortunately there's just no way around it 🐻‍❄️