Cobras appear in groups together or with Scorpions. They're found right in the middle of the game.
They are in the Dragon category, so the Dragon Sword (Wyrm Killer) works well against them in the WSC version.
Ever wonder why Cobras in this game aren't poisonous but Lobsters are? Cobras are actually Anacondas in the Japanese version (and FF Origins). Anacondas are large South American snakes that crush their prey rather than poisoning. They don't appear in any later FFs, although the English version of FF8 has something called an Anacondaur.