Learn English – What does unicorn mean in the following context

The Black Adder s01e03 – The Archbishop.
In the epilogue, Sister Sara asked the Mother Superior,

So presumably you won't be needing the unicorn tonight?

followed by a laughing track. Is she talking about the mythical creature or the carriage and what does the context mean accordingly?

Brief synopsis of the story:

The series is a sitcom set in medieval England. Prince Edmund (Black Adder) is made Archbishop of Canterbury after the
previous Archbishop is murdered by his father (the King). Almost
immediately, some drunken Knights come to murder him too because of a
misunderstanding of an overheard conversation.

Black Adder and his companions escape into the nunnery and
dress up as nuns to evade them. The knights follow them into the
nunnery and also dress up as nuns to be more subtle. The two groups
encounter each other and fight. They are soon disturbed and stopped by the Mother Superior who discovers their identities.

Black Adder tells the Mother Superior it was all a sordid sexual role-play and gets excommunicated so that he never has to be the archbishop again (and thus won't get murdered – at least not for that reason). The epilogue is the Mother Superior talking to Sister Sara. It is as follows:

Mother Superior: Alas the corruption of the world
Sister Sara: Yes alas Mother Superior
Mother Superior: I am tired and weiry. You may leave me now.
Sister Sara: Very well
[turns to leave]
Mother Superior: Alas
Sister Sara: So presumably you won't be needing the unicorn tonight?
Mother Superior: No… No not tonight