You have a wyvern mount

Wyvern mount

When you join this subclass at 3rd level you gain a wyvern mount. Add your proficiency bonus to the beast’s AC, attack rolls, and damage rolls, as well as to any saving throws and skills it is proficient in. Its hit point maximum equals your fighter level times 5

 The wyvern obeys your commands as best as it can. It takes its turn on your initiative. On your turn, you can verbally command your wyvern where to move (no action required by you). You can use your action to verbally command it to take the Attack, Dash, Disengage, or Help action even if your mounted on it. If you don’t issue a command, the beast takes the Dodge action. Once you have the Extra Attack feature, you can make one weapon attack yourself when you command the beast to take the Attack action

Bonded

When you use your second wind feature your wyvern may also gain hp equal to the number rolled on the die.

Mystical beast

Starting at 10th level your wyvern’s attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage.

Coordination

You have advantage on dexterity saving throws against falling of your mount. Additionally you can double your proficiency in animal handling for controlling your wyvern

Swift action

Your can use your bonus action instead of an action to command your wyvern. If your command your wyvern to attack you can use your extra attack to make you or your wyvern attack again.