The term "a doe in velvet" is real, and refers to a rare phenomenon in deer species where a doe develops antlers like those of a buck. There isn't a lot of readily available information on this topic, but I did find a post about a hunter who had shot one (mistaking it for a buck), complete with photos.
Liana rode through Elvenwood
With Vail, her brother, by her side.
She said, “I do not want to go
To Verdenholt as some king’s bride.”
“What would you do instead of this?”
He asked. “It is your destiny.”
“I’d learn to hunt, and live alone,”
Liana said, “if I were free.”
“Liana, dear, don’t be a plague,”
Her brother said. “Why, any maid
Would give her jewels to take your place!”
“Then let me go,” Liana bade.
Her brother merely shook his head,
So they continued on their way.
At Verdenholt, maids dressed her up
In velvet green and satin gray.
They walked with her through gardens tame;
Liana glided like a ghost.
It wasn’t beauty – she was plain –
It was her grace they envied most.
Liana flushed a hare. The maids
All scolded her. “This can’t go on!”
She laughed, and pricked her pointed ears
To follow where the game had gone.
The balls were boring, banquets dull;
She didn’t like the King at all.
She chatted with the hunters, though,
And guards off-duty in the hall.
The courtiers were whispering
About her ways, behind cupped hand –
“A doe in velvet, so I say!”
Liana didn’t understand.
She asked the maids, “What does it mean?”
“It’s not a term of which to brag,”
One said. “It means a doe misborn,
With horns attired, like a stag.”
“What Siela means,” said Calarie,
Is that those lords do not approve
Of lasses acting like to lads.
It means your manners best improve.”
The words caught fire in her head.
A doe in velvet – that I am!
Liana thought. I never knew,
And now I must escape this jam.
She cut one velvet skirt to breeks,
Then stole some bread and hid some fruit.
She left one night amidst a fog,
And dawn brought only cold pursuit.
She waded creeks and climbed up trees.
Guards hunted her to no avail.
Liana crossed the border road
Before they even found her trail.
She bound her breasts with satin scraps
Then stole a knife to cut her hair,
And “Lian” was the name she gave
To people headed toward the fair.
The Harvest Fair was full of men,
Not elves, all that she’d ever known;
But men had never sold her off
And Elvenwood she had outgrown.
A hunter down from Aspenwald
Took Lian’s service at his word,
And nevermore in any wood
Was lost Liana ever heard.
A forest’s full of stranger things
Than eye of elf or man has seen:
A salamander in the coals,
A doe in velvet in the green.
On Aspenwald’s secluded trails
Where silence almost seems too loud
And shadows steal the shapes of things –
There Lian walks, alone and proud.