Mounts can either be bought or taken from other players, or you can create one yourself.

To create a mount, you first need to find Steedgrass. This is a rare drop in herb gathering, and depending on your luck, it can take a while to get. Once you have the Steedgrass, you need to buy the Taming skill and a taming whistle. You can find these at NPC vendors.

Once you've created a mount figurine, it will appear in your backpack. To summon the mount, aim at the ground ahead of you where you want your mount to appear. Double-click the mount figurine in your backpack. Please keep in mind that anyone can run up to your mount and steal it unless you are riding it, so make sure you spawn it away from other players, and don't leave it unattended.

If you leave a your mount spawned and unattended, it will slowly start deteriorating until it dies.
If you lose connection, crash, or log out of the game while on a mount, the mount will stay behind in the world. Remember to quickly log back on to try to reclaim it.

To de-spawn a mount, walk up to it while aiming at it, and press alternate use (default G) to make it into a figurine again. The figurine will appear in your backpack.

If you die, you will drop your mount figurine for others to loot, so carry it with you only if you need it, or if you can afford to lose it. It's recommended you keep it safely in your bank if you're not using it.