One difference:
to summon different pets:
/pet 0, summon - summons 1st pet/mount (for me rivnak)
/pet 1, summon - summons 2nd pet/mount (for me yulbar)
/pet 2, summon - summons 3rd pet/mount (for me drifter)
/pet 3, summon - summons 4th pet/mount (probably pterosaur after they will be implemented)
to mount you must first target the familiar, then use /mount or use /mount <pet name>
to dismount use /unmount
I have this macro in shortcuts:
/target clear
/target next npc
/mount
other commands are same for all familiars
You can also use /pet name <name> for giving the actually summoned familiar/mount the <name> or use /pet <id number>, name <name> if that familiar is not summoned
it's also possible to target your pet by /target <pet_name>
/pet target = make it target an opponent (although I do not know for sure how to select one specific target)
it's easy, first target that specific target normally (mouse, /target next npc, /target next player...), then use /pet target to make your pet target it or use it as standard /target command i.e. /pet target next npc, /pet target <npc_name> and so on