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The trip through the slums can net you an egg which can hatch into one of four mons, and a pokemon boss fight. Some wild encounters are coded as trainer fights, the reasoning flavourwise is that they have no intention of joining you and it lets the decs give you fights against the wildlife without giving you an option to capture them. I have mixed feelings on this.
Scafty was a fucking amazing pokemon in my first playthrough and carried me hard, you can pick up one here later in the game.