Jtdcftul

I want to cast Find Steed. I don't want to take levels in Bard, Thief Rogue, Wizard, or Paladin.

I had the idea that I could True Polymorph into a creature that has the Use Magic Device ability, or access to the Paladin spell list, and then cast Find Steed off of a paladin scroll.

background reference: What creatures can cast spells from spell scrolls?

Question

Is it even viable in theory that a character without access to paladin spells uses True Polymorph to turn into a creature who can use paladin spells, casts Find Steed from a scroll and then retains the summoned steed afterwards?

An important follow-up question: what is an example of a published creature able to use paladin spell scrolls? I want to satisfy the case where the DM insists that you need to have a published creature of some sort in order to cast True Polymorph.

Yes, you can use those scrolls if the true polymorph form can

True polymorph states:

The target's game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality.

The class feature that allows the form to count as a paladin (or have the paladin's Spellcasting) for the purpose of magic items is included in the game statistics, so you gain them while under the effect of true polymorph.

This answer discusses why monsters with the Spellcasting special trait can use spell scrolls if their connected class spell list contains the spell on the scroll. The Introduction explains that this special trait works the same as the class feature in the Player's Handbook:

A monster with the Spellcasting class feature has a spellcaster level and spell slots, which it uses to cast its spells of 1st level and higher (as explained in the Player’s Handbook).

The monster has a list of spells known or prepared from a particular class... The monster is considered a member of that class when attuning to or using a magic item that requires membership in the class or access to its spell list.

One such monster for paladins is the Death Knight from the Monster Manual (you can use this D&D Beyond search to see all of the potential options, there may be some spoilers for adventure modules):

The death knight is a 19th-level spellcaster... It has the following paladin spells prepared:

Does a found steed persist?

The found steed will indeed persist and heed your commands as "you" have not become a different entity. You are merely assuming a different form:

If you turn a creature into another kind of creature, the new form...

However, the language the steed learns will have to be one that the new form speaks...

[the steed] gains the ability to understand one language of your choice that you speak.

since at the time where the steed gains the language, your ability to speak particular languages may change.

Yes, you can True Polymorph into a creature that can cast paladin spells to cast Find Steed from a scroll, but there are simpler ways to achieve the same thing.

Blackguards and Deathknights can cast paladin spells, so once you have polymorphed into such a creature, you can use paladin scrolls, and therefore cast Find Steed. You would retain the steed afterwards, as the steed only disappears when you either dismiss it or it drops to 0 hit points.

However, this is a really expensive way to cast Find Steed unless you're a 17th-level spellcaster or higher, as True Polymorph is a 9th level spell.

Another idea: Spell Storing.

This would achieve the same thing in a different way, and might be accessible to you long before you're able to cast 9th level spells like True Polymorph.

You'd need to find a Ring of Spell Storing or a Shield Guardian, and ask a friendly Paladin to cast Find Steed into the ring or guardian.

From the Ring of Spell Storing description:

Any creature can cast a spell of 1st through 5th level into the ring by touching the ring as the spell is cast. The spell has no effect, other than to be stored in the ring.

This would allow you to later cast Find Steed yourself:

While wearing this ring, you can cast any spell stored in it. The spell uses the slot level, spell save DC, spell attack bonus, and spellcasting ability of the original caster, but is otherwise treated as if you cast the spell.

Third idea: dip into Paladin

Multiclassing to take a single level in Paladin (you might not consider yourself to be "playing a paladin" at that point) will let you cast spells from paladin spell scrolls.

If the spell is on your class’s spell list, you can read the scroll and cast its spell without providing any material components.

You'll need to make a DC 12 Arcana check though:

If the spell is on your class’s spell list but of a higher level than you can normally cast, you must make an ability check using your spellcasting ability to determine whether you cast it successfully. The DC equals 10 + the spell’s level. On a failed check, the spell disappears from the scroll with no other effect.

Fourth idea: Wish.

Why cast True Polymorph and waste a spell scroll when using a different 9th level spell, you can achieve the same thing?

From the Wish spell's description:

The basic use of this spell is to duplicate any other spell of 8th level or lower. You don't need to meet any requirements in that spell, including costly components. The spell simply takes effect.

If you have access to 9th level spells anyway, why not simply wish for a steed?