If you're interested in selling back your mount for any reason, just hand the bridle back to one of the NPCs in the Stables. You don't use the trader interface, just hand it to them. If it's the wrong person, they'll hand it back to you. As said above, you'll get back a "bag of platinum". This you sell back the same way you buy and sell items to merchants. CHA buffs are always recommended. You usually lose about 1/3 of the money you paid for the mount.
For lizards, same thing applies except you're in the lighthouse in Gunthak handing back a drum.