So a long time ago I was playing Pokemon diamond. When Lucas was demonstrating how to catch a Pokemon, his chimchar was female. ( I still play diamond though I'm past the champion) and today when I started pearl, his chimchar was male. I'm deseprate to know.
When Lucas picks his starter Pokemon, it selects male or female. Because you were using two separate games (They don't have to be separate cartridges, just different save files.) it is possible for Lucas's starter Pokemon to be a different gender. However, all of Lucas's other Pokemon have set genders, so his starter Pokemon is the only Pokemon that he has whose gender can be different between different save files.
Hope this helps!