The red gyarados was only provided in game in pokemon gold/silver.
If you want a red gyarados in pearl/diamond, it's just random luck. Every majikarp or gyarados you see has a 1/8192 chance of being shiny, so keep looking, if you're that determined.
Edit: To the poster below me: I saw you post that exact message in another topic, but it has some flaws. First, you can't use the pokeradar to find a majikarp/gyarados, because they're not found in grass. Second, pokeradar streaks DO increase the chance of finding a shiny pokemon. Finally, refreshing GTS is NOT a good way to try to get the shiny you want, since usually shinies are dealt through friend codes (too valuable for GTS).
Finding shiny pokemon is completely luck. There's no specific location, but anywhere you can find a gyarados one might be. The chances are 1 in 8192, so you probably won't find one. If your really desperate keep refreshing gyarados at the gts you might find one, but be prepared to give up a great pokemon. If you were one land you could use the pokeradar to find one where sparkles were, but this doesn't increase your chances. In my 200 hours of diamond I've only seen 2 shinies.
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Verified answer
The red gyarados was only provided in game in pokemon gold/silver.
If you want a red gyarados in pearl/diamond, it's just random luck. Every majikarp or gyarados you see has a 1/8192 chance of being shiny, so keep looking, if you're that determined.
Edit: To the poster below me: I saw you post that exact message in another topic, but it has some flaws. First, you can't use the pokeradar to find a majikarp/gyarados, because they're not found in grass. Second, pokeradar streaks DO increase the chance of finding a shiny pokemon. Finally, refreshing GTS is NOT a good way to try to get the shiny you want, since usually shinies are dealt through friend codes (too valuable for GTS).
i have a short AR DS code for it if you want...
94000130 FCFF0000 B21C4D28 00000000 B0000004 00000000 E0000304 000000EC 23F023F3 A2C80000 DED7E00C 72FF77C4 D68FA129 3464A593 4ED64AEA 68BD5BF0 7CD6981F 191AC59C C85F14D4 D264FE24 B00C4107 79417333 16A6F1F6 30A25C30 DF7AD8B3 02235CBE 948F4167 8BD13C47 292C5F22 A6583A53 E6A2AC88 FDAC9D38 D9711F6A D8DDDFE1 91E5DF6B 2570C559 EA6D2E0E C63D397F 4A1F630D EB5233DC 1CE5DCD6 2D490AAA 0263F925 79A0232C A01454E0 BFEDD06C 225C390B B3E3CDDA 077F3448 54FDD8E1 D643D629 FF511225 177B2DA6 498437C9 66535393 2F61B3C9 59161171 62A41BA7 3028E3AA D5160649 8039DC1A CFD95C21 4BB728D1 2C04858D 3F844316 80721B81 06E90AAF 00000000 20000300 00000001 D2000000 00000000
And yes that is actually veeeery short...some of my codes are like this one here (below).
94000130 FCFF0000 B21C4D28 00000000 B0000004 00000000 E0000304 000000EC D4F5D4F3 F0E60000 396B009F 2397A579 CC0575DB 2B5F1EB1 A4B5C5D6 A5F118DC 07E05C2D C8717F50 F70201A2 F6AE8263 2B82802F E43A6750 D94E7A36 06144F45 80039DF6 5BC99F0F 622D9CDE D627BFDF 0AF4D511 DDAF4754 6DE66F24 42AC8AE4 9DBF40E4 F581BAE8 AAF10155 BB824205 771ED76E D7A66862 97410252 DDD94A8F CF69F910 2154C5D6 0C33FA80 33D8E72E DA886F0F 523BC2CD 53AA675C EA2AE15C 4FECEF03 5F51233A 13AB3555 F973B17B C608D06E CA45878F 1BBD1EF5 E9A2D1C6 CDA6BD3A D656BCEF D7B85777 103A9C41 B3325C57 3F80897F 001153BC 7D50E356 51C9DDBC 91924E08 13319DED 00000000 E00003F0 000000EC 44E544E3 D16F0000 0E516B6A B5FDB189 A93D6F20 AC3984AB FA58F016 634FB6A9 902CF66B 4D828615 971E22F0 E0A5A8EB 13117475 AB613460 76B862AF 7662938C 2C31D2C4 14E1AA98 6B0A2360 E8AF5738 7483B3B0 02B745E7 1BED1A60 E76DFDBE 66D30D6B 421B1D5C 80A4005F FD7C6234 482C4F91 85203233 261C6791 E54C72CD 34D0B946 3A6CCE70 B49E1B2D 28C9899C 1D7629A6 04211083 73E97C30 08CBFF7C CB8A5A90 2AB4888C 28F3BB5F A5577EA4 6AAB4663 1F68BACF C8037CD3 16B105A6 8EDD33C3 5FDD21F6 19A59961 CF93DDA0 30814A7A 60C64CC0 49C73C43 6C4A9311 5DAF7E56 C5AADF8F 414918FA 00000000 E00004DC 000000EC 88B388B3 E2740000 A77DDE9A C8CF8C02 685DA7CC 775F515D AC830534 4D7C2B3A E0BCDF0D E76F0298 E4ACB985 E9047B16 749F9EBD 8D686234 51E73B01 1FC4F1A9 C622C937 DC244658 D98D9828 D05FC336 5618B69D 40C6CBFF 64A78C96 024B3FC6 849EDC69 5F113D99 6581D0E8 9B7E895A 71A57771 6FFD0594 D6C74F40 3838BC0E 5C42FCD5 8F6731AE 22A3158A F5AF114F DED2E83F 2B143E6F 14C16F97 B26D3DF1 277BBBD3 0AC3E3D3 2E7D6253 5243302F 4B33806F 6DE08921 EDF3AD59 406F8464 A8F4BAAE C779E2F4 8B4892B4 3152A2F5 370AB6E3 406A4BD3 26D322A6 8FE2D8A9 D6729662 8D61E4E6 C2DB8EB6 00000000 E00005C8 000000EC 72DD72DB BDB30000 0DF265E6 6DB7FF23 B7C46756 8DBA9459 2E313AD7 9124F82A 16DBFB10 8E48EB7C D8D13BBD 82F89320 C62EAC04 3B214545 B6CB5FEE 91B9C373 AF3A2020 73854FBB B1C163E4 ABDD4096 EB0FD4A7 E197ECBC 518D50D9 BF0F9922 0F6C3317 32445658 F5536704 84C6E091 8371B2D0 8B11E83F 927783EE 79DB98FC D6C84E51 8A54DA37 E200D1AB F822BE2F 120A7BAF 58941B9E BD58CB15 46524BC8 BFF86348 BA70BBF5 9BA9B196 7AFC0CEB 6B4708ED 408CCF62 2DCE9FAE 994010F9 8805E92D D813E459 75F31473 A574479A 9F91B33F 05140CE4 44C0CD4E E501A959 F47CE3DB 89F30E32 8352A969 00000000 E00006B4 000000EC 00B000B3 B4730000 AD1BFBC3 AFF3739C FCB0DC50 C96AFE7D B49C03E2 485855C6 18E3C112 98937875 4B643BC4 E04FF721 EB15E77F 430F1C6F 69C29B7B 59A0E51B 5273C247 0947FBC8 6A6234B5 8F663635 688DFD22 9CF51168 8F68464D 21C67FFC D2E94E7D 77AACAD8 4465DB25 263F4CEC 900FB535 7EB73825 DD57CC9E B017C5AD 3D441854 0688CF87 2960E899 FA4BE6A8 81F5F857 27914819 1FA481C9 E8405BD5 1432BE37 16CEF81C 5AF88358 207DFBB6 B02AC8AD B1C2023E 579D16E9 6CF0AEEE 6F0AC928 87122844 C9026B2F 5DFAA9E2 831FB7F7 5B9950B5 CB7BFC6C BA918559 8965E271 0008A8BC DD52E827 00000000 20000300 00000005 D2000000 00000000
of which gives me my 5 favorite pokemon
Finding shiny pokemon is completely luck. There's no specific location, but anywhere you can find a gyarados one might be. The chances are 1 in 8192, so you probably won't find one. If your really desperate keep refreshing gyarados at the gts you might find one, but be prepared to give up a great pokemon. If you were one land you could use the pokeradar to find one where sparkles were, but this doesn't increase your chances. In my 200 hours of diamond I've only seen 2 shinies.
there is no specific place other then in water
it is a 8,192 chance to find a colored pokemon and you can't pick and choose which ones you find without action replay