Some more world building to consider

[In a reply to a comment about a unicorn reacting with mild nausea to Pegasi seafood by saying they are too isolated to the wider world around them]

Sheltered? But yes, there's almost always that reaction to cuisine outside standard vegetarian fare. For most readers here, to get the same reaction, you'd have to resort to something exotic like deep jungle fare-- eyeballs and ox gonads. I am a little surprised unicorns haven't got something unique to themselves in a food table way; earth ponies tend to be strictly vegetarian, pegasi have some seafood, but unicorns don't have something to call their own.

Maybe something magic based would be it? A spice or sauce that's literally "magically delicious" [have to sing that part] added to or a side dish of the unicorn meal. A ubiquitous spice sounds like a decent world building idea and easy to incorporate. Might taste weird but have no effect on non-unicorns while those with a horn would find it irresistible and rejuvenating, the spice making food process more readily into energy replenishing mana for them and in higher doses, a mildly euphoric sensation like a sugar rush. I suppose you could go the whole "Dune" route and say that eating just pure spice would cause eyes/horn to glow, lips to stain, and some possibly degenerating effect with too much mana being absorbed for the unicorn to handle (can't become an alicorn that way despite the legends). To prevent that, it would need to be rare in the wild, something like Saffron comes to mind; the most expensive spice on Earth, it takes almost 500 hand picked stigmas to make a single gram of the spice (it comes from the stigma of the blue flowering crocus). However, it's extremely potent and just a tiny amount will add color and zest.

To make it more unicorn based, it could be assumed that trying to harvest by hoof is a disaster, requiring a delicate telekinetic touch to remove the stigmas without obliterating the delicate flower itself between clumsy hooves-- like trying to pick up an eye contact from the ground with boxing gloves. A pony such as Rarity would be ideal with her precise TK magic. Yes, a non-unicorn could do it with tweezers but they didn't have those back in the early pre/post-unification days so it's far more traditional for unicorn harvesters to do it and ponies do so love their traditions.