Encountering Delgado by chance is INCREDIBLY rare, but there is a process to garuntee that he’ll show up. This process however also requires a lot of chance.
There are three pokemon in the game that do not show up in the pokedex. The first one is Sandile. Seem familliar? This first secret pokemon actually got implemented later in gen 5. The first step to encountering Delgado is to find and capture a sandile. You can find them in the desert, but they’re rare. Really rare. Roughly 30% rarer than shiny pokemon. That cannot evolve in this gen. Sorry no krookorok. Make sure you get a male one. Use a female pokemon with cute charm as your lead to increase the odds.
Step 2: In gen 3, sandile is coded to be incapable of breeding UNLESS it is a male sandile and a female mudkip. These eggs will most of the time hatch into more mudkip, but there is an extremely low chance of the egg hatching into the second secret pokemon Mudile. Mudile was likely inteded as a prototype to gamefreak’s scrapped “Hybrid Pokemon” project. Anyways, once you finally get a Mudile, level it up to level 11 so it learns it’s signature move, sand slap. This secret move has a modest 70 power, but it DOUBLES the multiplier when hitting supereffectively. Pokemon like Aggron, Bastiodon and Stakataka cannot stand to such a move. Please do not repeat my mistake in evolving it, it only evolves into Marshtomp and sand slap becomes mud slap. Having to get another Mudile is a huge setback...
Once you get a Mudile with sandslap and the Walrein that knows snore (MAKE SURE IT IS ASLEEP!) in your party, Delgado will show up in the desert garunteed. Talk to him and he will challenge you to a battle. He specializes in flying type pokemon, but also has a Scizor. This will be a grueling fight, but once you beat him, he’ll reward you with the third and final secret pokemon, Stingulator.
