Here's a simple long-term strategy I usually suggest: make space in your stable for pets that can do quests. Quests are your easiest way to get money over time. Focus on the simpler quests, like searching the pound for a pure black puma or striped black dragon. A lot of people do put these in the pound, so it's a start.
Then when you find a quest that happens fairly often, try to buy a pet that fulfils that quest, like a tan forest throdama, or a fiery red symurgh. You can get them fairly reasonably priced, if you keep searching. You'll soon know how much is a good price (at least for you).
Remember that each pet that you have will cost money for food, if you don't explore that day. But any pet that can do a quest can pay its own way. That's why a large chunk of my stable is just questers. I've put a lot of the pets that I bred in the first year I played this game into my Reserve, but questers I try to keep or sell on, because they're so useful. They more than pay room and board. Even if you can get hold of a pure white symurgh or a blue symurgh from out the pound, you'll get a few thousand silver every time they fulfil a quest.
In general, the more you quest, the more money you have. Try to save up and buy good fighting pets as well as questers. I spent most of last summer (so northern hemisphere's winter) using one gp3 Lykos with ice breath which I slowly levelled up to 50 to explore and fight some really impossible beasts. I saved up for her mother and bred her from a really good father as well. Breeding really helps. You'll start by caring a lot about colour or one particular pet, but eventually you'll get a feel for what's really good to have (and while it isn't easy, it is possible to breed yourself some really nice pets).
If you need any help, just let us know :)
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