Has Leta Lestrange been mentioned at all outside of the Fantastic Beasts and Where to Find Them movie?
This article in Bustle, entitled "Is Leta Lestrange In The Harry Potter Books? Newt's Mysterious Ex Is A Brand New Character", makes this very clear:
In Fantastic Beasts, the Lestrange family doesn't directly appear, but it is teased that Newt Scamander had a close relationship with one Leta Lestrange (played by Zoe Kravitz). If you're like me, the second you heard that name you thought to yourself, "I don't remember a Leta Lestrange." And if you then panicked about possibly forgetting a Harry Potter detail, don't worry: Leta Lestrange is not in the Harry Potter books.
Given the fact that Leta is a new character to the Harry Potter world, there isn't much we can learn about her from the Harry Potter series, but we will be learning a whole lot more in the planned Fantastic Beasts sequel. "Leta comes into the second movie," director David Yates said at a recent press junket for Fantastic Beasts and Where to Find Them.
Both of the links in the quoted text above are to a second Bustle article, which confirms that Leda [sic] Lestrange is a brand-new character created for the Fantastic Beasts films.
JK Rowling was asked a similar question on Twitter:
who is leta lestrange plz i can't live like this
J.K. Rowling: Next movie.
This would seem to imply that she hadn't been previously mentioned.
Additionally, I can confirm (by means of doing computer searches on Rowling's previous writings) that no character with that name has appeared.