What it is
'Natural flavour' is not one ingredient but a legal category, a catch-all for concentrated tastes drawn from natural materials rather than built from scratch in a laboratory. The US definition is worth reading slowly, because it says a natural flavour is the oil, extract or other product whose flavouring parts come from a long list of sources: 'a spice, fruit or fruit juice, vegetable or vegetable juice, edible yeast, herb, bark, bud, root, leaf or similar plant material, meat, seafood, poultry, eggs, dairy products, or fermentation products thereof'. Read that list again and you will spot meat, seafood, poultry, eggs and dairy sitting quietly among the herbs and fruits. The single rule that ties them together is only that their 'significant function in food is flavoring rather than nutritional', not that they come from a plant. So while the great majority of natural flavours are plant extracts or the output of microbial fermentation, the category is legally wide open to animal ingredients, which is exactly why a blanket 'natural flavours' cannot be assumed vegan.
Why it trips people up
The trouble is that on the label the whole category collapses into two or three words, 'natural flavours' or 'natural flavouring', with no breakdown and no source, partly because a flavour formula is treated as a protected trade secret. That black box is where the surprises live. The most talked-about is castoreum, a substance taken from the castor sacs near a beaver's tail, which a published safety review confirms has been 'added to food as a flavor ingredient for at least 80 years' and recognised as safe for that use, lending vanilla and raspberry notes. In real life castoreum is now vanishingly rare and used in tiny amounts, and the far more common animal-derived flavours are ordinary things like dairy, egg or meat extracts sheltering under the same generic phrase. Because the word alone can never tell you, the practical route for a vegan is to treat an unspecified 'natural flavours' as a question rather than an answer: trust products carrying a registered vegan trademark, which have already checked, or ask the manufacturer directly.