Heartfire Hero and Emberheart Challenger stand out as the mice that actually crack that top tier themselves, even in a deck not wholly designed around generating valiant triggers
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While tutoring for any spell is primarily found in black cards, green has a significant number of cards that can tutor for creatures, often at a better rate than burning a Demonic Tutor on them
Still, it can be flipped most of the time
If you wanted extra cards playing mono green there were usually drawbacks like needing an artifact that benefited all players, such as Howling Mine
Would you play with a five-mana sorcery that read, Deal four damage to target creature, deal four damage to target player, and put a 4/4 flying dragon token into play tapped at the beginning of your next turn?