Farming Deviate Fish
Deviate Fish are used as a material by Cooks, who make it into Savory Deviate Delight, or by Alchemists to create Elixir of Giant Growth. While neither of these items grant any benefits in combat, you would be surprised how much gold you can make by selling these. Fishing can be great to make gold.
I recommend a combination of the secondary professions Fishing & Cooking, paired with the primary professions Alchemy & Herbalism. This can be a very vialble way to make gold in classic WoW.
Fishing for buff food
Many of the high-end foods that Cooks can create require fish. Of course, it would be a lot more lucrative if you had Fishing and Cooking. You can make some decent gold just by fishing these high end fish and selling them:
Raw Nightfin Snapper
Fish for Oils
Some fish are also needed by Alchemists to create oils like the Stonescale Oil. Alchemists need these oils for the creation of Flasks and Potions, such as the Free Action Potion. Apart from the Stonescale Eels, the following fish can be farmed at lower levels, too. It’s a good way earn gold for your first mount.