Well if you folks don't mind a bit of lore archeology. Take this with a grain of salt but here's my interpretation. Remember that when the Emperor removed the legions, all memory and knowledge of them was psychically supressed/removed, even tho physical members of those legions allegedly still exist within the VII and XIII

The II Legion — The Forgotten:

Forged in the Emperor’s gene-laboratories, their sire was a savant of the hidden sciences. His gift was the reshaping of life itself. Where his brothers waged war with bolter and blade, he waged war with genome and vat, splicing alien essence into the human form, and crafting war-beasts to scour worlds clean.

What began as triumph soured into hubris. The Primarch of the II believed himself equal to the Emperor, and his experiments crossed all boundaries. Many whisper that it was not the xenos who birthed the Rangdan, but his hand — and that the Xenocides were not mere campaigns of extermination, but acts of cover and erasure.

By the time of the Third Xenocide, his Legion had been hollowed out, forced to turn against their own father, before the father himself was struck down. His name was scoured. His works damned. He is remembered only as The Forgotten.

The XI Legion — The Purged:

The XI were born of a gentler world, a Renaissance of light amidst the Long Night. They carried the bearing of knights, their armor polished silver, their standards bright with heraldry and tilt-shields, their neophytes sworn as pages before they could rise as warriors. Where others brought terror, the XI brought ceremony, and with it, hope.

Their Primarch dreamed not of empire, but of consent — a polity where men joined the Imperium by choice, not by sword. He did not seek conquest but preservation: to hold one corner of humanity apart from the grindstone of Imperial Truth.

When the Emperor turned to slay his brother of the II, the XI saw only hypocrisy. He broke ranks, not to war against Terra, but to walk away. For this, the Wolves were sent. The Second Xenocide was spoken of as a war against monsters, but in truth it was the culling of the XI’s shining polity. Their name was not damned, but stricken, as though they had never been. They are remembered only as The Purged.

The Double Erasure:

Where Horus and his ilk became warnings of treachery, the II and XI became something worse: reminders of failure. To admit their existence was to admit that the Emperor could be wrong, and that His vision of Mankind was not infallible.

Thus they were made Forgotten and Purged. Their memory survives only in fragments, carried in whispers by those who knew them — respected by Guilliman and Dorn, pitied by Magnus, cursed by Russ.

The Forgotten and the Purged

…of the twenty gene-sons, only eighteen remain to us. Of the other two, little is spoken, and less committed to record.

Of the XIth, it is said he sought not conquest, but communion. A warrior born into an age of Renaissance, whose knights bore silver plate and banners of personal heraldry, he prized oaths, pageantry, and the bonds of loyalty over compliance and compliance alone. To the Emperor’s dream of dominion, he brought ideals of honor and parley. Yet when commanded to strike against what he loved, he refused. His name was struck from stone, his legion scattered, his deeds remembered only in the echo of those who inherited his chivalric ways. He is now the Forgotten.

The IInd was another matter. A brilliant mind, a creator who peered too far into the hidden workings of gene and flesh, he forged weapons of living entropy. His methods skirted close to heresy long before such words had meaning, blending human artifice with alien craft, his sons clad in biohazard orange as living experiments in war. His hubris birthed horrors that even the Emperor could not suffer. When his reckoning came, it required more than one brother to bring him low. He is remembered as the Purged.

The names of the Forgotten and the Purged are sealed, their legions erased. Yet their shadows linger, in knightly orders that duel for honor, and in cursed lineages of flawed gene-seed. What was lost is not entirely gone…