Thankfully, it’s possible for trainers to trade Legendary Pokémon with others in Pokémon GO — but unfortunately, it comes with a few caveats that make it a bit more difficult than your average in-game trade.

For starters, trading Legendary Pokémon is considered a special trade, meaning it’ll cost you and your trading partner more stardust to complete the transaction. If you don’t have enough stardust, you won’t be able to complete the trade.

Source: https://www.distractify.com/p/can-you-trade-legendary-pokemon-pokemon-go