The Unicorn like the Phoenix is a singular creature, meaning there's only ever one (traditionally anyway, I'm sure there's been contemporary herds of the things roaming our novels and TV screens).
The Maiden and the Unicorn by Domenico Zampieri Domenichino, c. 1602.
The Unicorn in Captivity by an unknown Flemish Weaver, 1495-1505.
Sadly, due to the Unicorn's uniqueness and it's horn's supposed healing properties, this creature is the source of many a hunt and quest.
Chastity with the Unicorn by Francesco Di Giorgio Martini, 1463.
It was said that the Unicorn could only be captured by a virgin (female of course, for some reason virgin males just don't have the same effect) who was basically put out as bait. The poor Unicorn would come lay it's head on her lap and fall asleep leaving itself free to be captured by the waiting huntsmen.
A Unicorn from the Spiderwick Chronicles.
A Mon Seul Désir by an unknown French weaver c. 1410.
Abduction of Proserpina on a Unicorn by Albrecht Dürer, 1516.
St Justina with the Unicorn by Moretto da Brescia, c. 1530.
The patron saint of Padua, St Justina is sometimes depicted with a Unicorn to signify virginity.
Unicorn from Conrad Gesner’s Historiae Animalium, 1551.
Lady with a Unicorn by Raffaello Sanzia, c. 1505.
That is one little Unicorn.
The Unicorn is found from The Hunt of the Unicorn tapestry, c.1495-1505.
Allegory of March – Triumph of Minerva and sign of Aries by Francesco del Cossa, c. 1468-70.
The Lady and the Unicorn by Luca Longhi.