Champions of Kamigawa has a single cantrip, Squelch
For example, if you have no cards in your graveyard as it enters and you mill three land cards and a card with mana value 4, Monstrous War-Leech is a 4/4 creature entering the battlefield
Adventure is a pretty dumb mechanic, and this card has shown up across various Magic formats that want some extra utility out of its 1-drops
While Starmies 90 damage is quite good for two Energy, it wont one-shot bulkier foes, which means your opponents will be retreating quite often, and Articuno exs capability to KO foes even after theyve switched out is a great counterplay to this
Take, as a very good counter-example, Yuriko, the Tiger's Shadow
This precursor to the baby rule in Neo Genesis could certainly grind games to a halt, so that might be the reason for the rarity spike and high dollar value