Why Does Shedinja Appear When Nincada Evolves in Pokemon Emerald?

When Nincada evolves in Pokemon Emerald, it always becomes Ninjask. However, under specific conditions, Shedinja also appears at the same time.

This additional Pokemon is not shown during the evolution process and is not clearly explained by the game.

Why This May Cause Confusion#

During evolution, the game only shows:

Nincada evolving into Ninjask
A standard evolution animation

There is no indication that another Pokemon will appear.

After the evolution ends, players may suddenly notice:

An extra Pokemon in the party with 1 HP

Because this behavior is never mentioned, it can create the impression that:

The game has generated an unexpected Pokemon
Something unusual or unintended has occurred
The evolution result does not match what was shown

What Actually Happens#

In Pokemon Emerald:

Nincada evolves into Ninjask at level 20
If there is an empty slot in the party
And at least one Poké Ball is available

Then:

A Shedinja is automatically created
It is added directly to the party

Shedinja is not a separate evolution choice.

It is generated as part of Nincada’s evolution under these conditions, even though the game only shows Ninjask during the evolution.

Shedinja appearing in the party after Nincada evolves in Pokemon Emerald

In Short#

Nincada evolves into Ninjask.

If there is space in the party and a Poké Ball is available, Shedinja also appears at the same time, even though the game does not explicitly explain this behavior.