The more pricey ones are WW, mare and jet black mare, may sell for upwards of 50-200M depending on pet and stats. Regular dragons not so much at all. Before you sell any of them check on Stratics "Traders Hall", and find a thread with "Pet for Sale" click on that and will pull all threads with all pets sold and hopefully some prices. Worst thing you can do with these is sell them without knowing present day pricing, or even worse : Train them without any experience on training and what training does. Then their value is zeroed out pretty fast.
Also make sure the "pure black mare" is not a Dread War Horse renamed mare. Say the word "trick" when you are close to it, and if it has an explosion of smoke visual it is a Dread, worth close to 1 Billion uo gold = 1 Plat or so. Dreads have damage profile that is also different from regular mares. You can see stats of all these pets at uo-cah.com, lots of useful info on the game there too.
If you are really interested in selling you will need to use the cutting tool, and lore each one, and post ALL their lore pictues on the Traders Hall, and there ask for prices that could be offered i.e. PC = Price Check. You may also find byers there. Don't get ripped off, it is relatively easy to get ripped in game, esp. by folks pretending to be your friend.
Good luck and welcome on board.