If it's not Feebas, Special = Phenomena. Bubbling water/rustling grass/shadows/dust clouds in Unova. There are only 2 of these on the server, they exist for 10 minutes, then despawn and spawn in a new location after 5 minutes to last 10 minutes again. In return for them being rare you get better loot inside — a rare mon (some are exclusive to these) with higher than normal average stats and either a fossil or a mysterious gem. These gems can be combined with any type enhancing items, like charcoal, to get 5 consumable typed gems that provide a one time +50% boost to a move. You can also use 20 of these typed gems to change the type of Hidden Power a mon has to that type (doesn't need to know the actual move for this, it's permanent till you change it again or reset with normal gems).
https://forums.pokemmo.eu/index.php?/topic/77087-phenomenon-locations-guide/

Watch the global chat, add "pheno" to your chat alert. If you find one, announce it, please.
https://forums.pokemmo.eu/index.php?/topic/114920-easyish-guide-to-get-calls-for-phenos-from-chinese-players/

For Feebas — it's catchable in 6 spots across the whole river, so pretty hard to find. These spots change every 45 minutes. Some people organize raids to brute-force the river semi-regularly.