A fish or creature with a golden-colored mouth, or possibly a plant with golden flower parts.
Compound word from 'golden' + 'mouth' (Old English 'muð' from Proto-Germanic 'munthaz'). This appears to be a descriptive naturalist term, similar to other animal names like 'bigmouth' or 'smallmouth'.
Fish and animals often get named by early naturalists based on their most striking features—'goldenmouth' would have helped people identify and remember a specific species in oral traditions!
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