珍稀 is the standard adjective for endangered species and museum-grade rarities - typical collocations:
珍稀动物 (rare animal),
珍稀植物 (rare plant),
珍稀文物 (rare cultural relic),
珍稀物种 (endangered species). It bundles 'precious' (
珍) with 'rare' (
稀), so it always implies value, not just scarcity.
