Early in Harry Potter and the Half-Blood Prince, Harry observes Nymphadora Tonks casting the Patronus Charm:
As Harry swung the cloak back over himself, she waved her wand; an immense silvery four-legged creature erupted from it and streaked off into the darkness.
"Was that a Patronus?" asked Harry, who had seen Dumbledore send messages like this.
- Harry Potter and the Half-Blood Prince (Chapter 8 - Snape Victorious)
Due to a comment by Snape, Harry determines that Tonks's Patronus must have changed forms recently, and he asks Lupin about it:
"Tonks’s Patronus has changed its form," he told him. "Snape said so, anyway. I didn’t know that could happen. Why would your Patronus change?"
Lupin took his time chewing his turkey and swallowing before saying slowly, "Sometimes … a great shock … an emotional upheaval …"
- Harry Potter and the Half-Blood Prince (Chapter 16 - A Very Frosty Christmas)
However, at no point does anyone identify what Tonks's Patronus used to be.
What form did Tonk's Patronus take before it changed?