My guess is you clicked on the Zigzagoon right before it was about to despawn and your nephew opened the game to the Gulpin that spawned in its place. If you both had your games open at the same time and one showed a Zigzagoon and the other showed a Gulpin up at the same time, in the same spot, then that does sound a bit odd.
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Pokémon spawn glitch?
There is a single spawns point next to my home. This morning, the first spawn I woke up to was a zizagoon. My nephew also started his game and his spawn was a gulpin? Wondering if this is a new glitch and if others have had this happen.
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We have 3 spawn points in our house, one on 48 minute, 52, and 57 minute of every hour. This happened to my kid and I, we saw a specific pokemon, it turned to something else when caught, and they weren't dittos. One time I caught a high IV and cp eevee, by the time my kid and wife clicked on it, it reverted to low cp and IV. We were laughing on how niantic and their spaghetti codes.