Hey,
I've got a creature inside a leveled list.

This creature is classified as a mount, but

1) my character woun't mount it
2) it seems like it attacks everyone
3) I want just to make one of these be "friendly" to me, while all the other should just be hostile to me.

How do I do all these three steps?

What I would do is clone the existing Horses in the game and edit that to be the creature you want as a mount. Then it should behave the same as the regular horses in the game.

Try this:

1) Open the console with ~

2) Click on the creature (if done right, the creature name and reference will appear on the screen, at the top)

3) Type "ModDisposition Player 100" without quotes and press enter. This should prevent the creature from attacking you.

4) If the creature is already in combat with you, type "StopCombat" and press enter.

5) Close the console with ~

If the creature still attack you on sight, try type "SetAV Aggression 5" and press enter (followed by another "StopCombat")