Account-wide items like the account-wide version (not the less expensive per-character version) of the Steed of Rivendell, pre-order cosmetics, questpack unlocks and the currency cap removal are available for ALL your characters, no matter which server they're on. If you have several accounts you play on, you will have to purchase the account-wide item for each of them, though.
Regarding the account-wide Steed of Rivendell, all your existing characters should have received the mount skill and the War-Steed cosmetics traits upon their first login after you made the purchase. Any new character you may create in the future on any server will have a Steed of Rivendell item (which looks like a horse's head on a blue background) added to their inventory when you first enter Middle-Earth with them.
Once your character has acquired the riding skill, you can then right-click this mount item to transform it into a mount skill and a set of War-Steed cosmetics traits (you'll see the latter scroll by in your chatbox). The mount item itself gets consumed by this. For your next character, a new mount item will be generated (and you can have several characters with the same kind of mount item in their inventory, don't worry about that).
What cannot be done is to create a character to receive the mount item and send it do a different character on any account (including your own one) because AFAIK mount items are always bind-on-acquire and autoconsumed to boot if the character is able to use them.
German, female, migrated from CodeMasters in 2011, Premium member, slow-leveller, pure PvEer and opposed to Forced Emotes - wherever I fit into the statistic, it's not in its middle.