Ple-Sen
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Kosha, the Raptor Gold isn't valuable because it's particularly strong (the equivalent bird for Haven is Bronze and costs 1), but because of its spellcasting ability. And the effect isn't necessarily limited to just one bird. A better analogue to the Frog would be the Trinity Dragon, which costs 3, has 2/2 stats, cannot be attacked, and... is a Bronze creature. Notably, the Trinity Dragon isn't considered a particularly useful card.