Unlockable: Elekid
Insert Fire Red in the GBA slot.

GBA Cartridge Extras
Unverified
. Insert the GBA cartridges to find the following Pokemon (you still need to find and capture them yourself).

Ruby

Seedot
– Hakutai Forest, Route 203, Route 204
Nuzleaf
– Route 229
Mawile
– Iron Island
Zangoose
– Route 208
Solrock
– Thrust Lake, Thrust Lake Surroundings, Wisdom Lake, Aspiration Lake, Tengan Mountain, Departure Springs, Return Cave

Sapphire

Lotad
– Route 203, 204, 205 North, 212 South
Lombre
– Route 212 South, 229
Sableye
– Iron Island
Seviper
– Route 208
Lunatone
– Thrust Lake, Thrust Lake Surroundings, Wisdom Lake, Aspiration Lake, Tengan Mountain, Departure Springs, Return Cave

Emerald

Pineco
– Hakutai Forest, Route 203, 204, 229
Gligar
– Hard Mountain, Route 206, 207, 214, 215, 227
Shuckle
– Route 224
Teddiursa
– Route 211, Wisdom Lake
Ursaring
– Wisdom Lake, Route 216, 217

FireRed

Caterpie
– Route 204 South
Metapod
– Hakutai Forest
Ekans
– Route 212 South
Arbok
– Safari Zone
Growlithe
– Route 201, 202
Elekid
– Route 204 South, Powerplant

LeafGreen

Weedle
– Route 204 South
Kakuna
– Hakutai Forest
Sandshrew
– Lost Cave
Sandslash
– Route 228
Vulpix
– Route 209
Magby
– Hard Mountain, Route 227

Any Pokemon GBA Cartridge

Haunter
– Forest Mansion
Gengar
– Forest Mansion Room with eyes

Unlockable: National Dex
Finish the Shinou Dex.

Unlockable: Pal Park
Acquire the national dex.

Unlockable Legendary Pokemon Heatran
This legendary can be male or female but cannot breed. Get your national dex and go to the island in the north and east. When you get to Stark Mountain, you must team up with someone to go through the place. The trainer will take the item and depart. Head back to the Survival Area, and your “partner” will be inside a house. Talk to her and shell leave.
You have only one chance to capture Heatran, so save the game first. Return to the mountain, and Heatran will be waiting. The pokemon is at level 70. Heatran can learn the following moves:
Level 1 – Ancientpower
Level 9 – Leer
Level 17 – Fire Fang
Level 25 – Metal Sound
Level 33 – Crunch
Level 41 – Scary Face
Level 49 – Lava Plume
Level 57 – Fire Spin Level 65 – Iron Head
Level 73 – Earth Power
Level 81 – Heat Wave
Level 88 – Stone Edge
Level 96 – Magma Storm

Unlockable Eevee
Get an Eevee through one of two ways:
#1 Receive an Eevee from someone named Mizuki in Hearthome City
#2 Catch yourself a wild Eevee in the back of the Pokemon Mansion

Unlockable: Black Trainer Card
You must win 100 times in a row at the Battle Tower.

Unlock Manaphy (Pokemon Ranger)
Very many pokemon fans have wanted a chance to capture one of the rarest pokemon in sinnoh MANAPHY, but due to the fact that pokemon diamond and pearl do not have any availible manaphy in the ENTIRE GAME. So the easiest way for you to get a manaphy is for you to get the game Called POKEMON RANGER. This game has a mission that is available for you to play after you do a few things.
(1)
Beat the Pokemon Ranger game.
(2)
On the ranger net screen hold the buttons R, X, and the D-PAD LEFT (if you beat the game you will know what the ranger net is).
(3)
When the screen says enter password, input P8M2-9D6F-43H7
(Cap-sense is on).
(4)
Then at the special missions screen you will see a mission called RESCUE THE EGG. Tap on it then do the mission and get the egg, this will not be easy and will take a lot of time and effort but it all pays off at the end.
(5)
Borrow (or buy) a second Nintendo DS and the copy of Pokemon Diamond or Pearl you want the Manaphy on. The second DS should have Pokemon Ranger inserted. Go to the Ranger net with the Ranger DS and tap on CHECK THE EGG and then tap SEND MESSAGE. If your Diamond/Pearl DS is close enough to receive the message, you will get the Manaphy egg traded to you.
Addendum:

1. Turn on your other DS with the D/P cart.
2 Select the option to transfer Manaphy.
3. Do the wireless oks and what not.
4 Go to the nearest mart with 5 pokemon or less to pick up your egg.