Mistbreath Elder

At the beginning of your upkeep, return another creature you control to its owner's hand. If you do, put a +1/+1 counter on this creature. Otherwise, you may return this creature to its owner's hand.

Bloomburrow

At the beginning of your upkeep, return another creature you control to its owner's hand. If you do, put a +1/+1 counter on Mistbreath Elder. Otherwise, you may return Mistbreath Elder to its owner's hand.

Where you are is temporary. What you do is forever.
—Frogfolk saying

Bloomburrow Promos

If you control creatures other than Mistbreath Elder as its triggered ability resolves, you must return one of them to its owner’s hand. If you don’t, you have the option to return Mistbreath Elder to its owner’s hand or leave it on the battlefield and have the ability do nothing.