Wildborn Preserver

Flash (You may cast this spell any time you could cast an instant.)
Reach (This creature can block creatures with flying.)
Whenever another non-Human creature you control enters, you may pay {X}. When you do, put X +1/+1 counters on this creature.

Foundations

Jumpstart 2022

Flash
Reach
Whenever another non-Human creature enters the battlefield under your control, you may pay {X}. When you do, put X +1/+1 counters on Wildborn Preserver.

Throne of Eldraine

Throne of Eldraine Promos

If you pay {X} while Wildborn Preserver's triggered ability is resolving, the reflexive triggered ability triggers and will resolve separately. This is different from effects that say “If you do . . .” in that players may take actions after you've paid mana but before the creature gets +1/+1 counters.