Lair of the Hydra
Land
If you control two or more other lands, this land enters tapped.
: Add 
.
: Until end of turn, this land becomes an X/X green Hydra creature. It's still a land. X can't be 0.
Johannes Voss
Rulings
If Lair of the Hydra enters the battlefield at the same time as one or more other lands, it doesn't take those lands into consideration when determining how many other lands you control.
(wotc • 2021-07-23)
If you turn Lair of the Hydra into a creature but haven't controlled it continuously since your most recent turn began, you won't be able to activate its mana ability or attack with it.
(wotc • 2021-07-23)
Printings
Rulings
If Lair of the Hydra enters the battlefield at the same time as one or more other lands, it doesn't take those lands into consideration when determining how many other lands you control.
(wotc • 2021-07-23)
If you turn Lair of the Hydra into a creature but haven't controlled it continuously since your most recent turn began, you won't be able to activate its mana ability or attack with it.
(wotc • 2021-07-23)