You have to breed that Pokemon or make it evolve. To breed, use the poke radar to get a ditto in the grass just to the right of canalave city.
If you're not using AR then you have to Migrate Ho-Oh from GBA games. When they migrate, you will have to go to the Pal Park in diamond to catch all your migrated Pokemon. You can transfer up to 6 Pokemon a day
Dialga
You can get a Cyndaquil without trading only by transfering it from a GBA Pokemon Game onto your Pokemon Diamond Version.
no
no
i have Pokemon diamond, and played 600 hours. its not possible. but you can catch a ditto, and make Pokemon eggs.
you cant get it without migrating.
You probably caught it at pal park. This means that you migrated it from ruby/sapphire/emerald version. There is no way to catch a mudkip without cheats in Pokemon diamond. I'm sorry to say that it is impossible to catch in Diamond without migration or cheats: This is the guy who found torchic (not in pal park). I found it licking honey off a tree.
trade with Pokemon Platinum
you do need cheats
Rival
You can't without cheats.