Following up on information from comments (and elsewhere): for reference (from here ), the quotation from LW&W is
There was also a unicorn, and a bull with the head of a man, and a pelican, and an eagle, and a great Dog.
tl;dr Lewis was a scholar of medieval literature, wrote essays about Dante's work, and was also influenced by Dante in his other fictional work. It is extremely likely that he would have been familiar with Dante's version of the minotaur, which arguably was lower-half-bull, upper-half-man.
From Wikipedia, HT @Valorum:
Ovid's Latin account of the Minotaur, which did not describe which half was bull and which half man, was the most widely available during the Middle Ages, and several later versions show a man's head and torso on a bull's body — the reverse of the Classical configuration, reminiscent of a centaur.[17] This alternative tradition survived into the Renaissance, and is reflected in Dryden's elaborated translation of Virgil's description of the Minotaur in Book VI of the Aeneid: "The lower part a beast, a man above/The monument of their polluted love".
According to Borges's The Book of Imaginary Beings,
Dante, who was familiar with the writings of the ancients but not with their coins or monuments, imagined the Minotaur with a man’s head and a bull’s body (Inferno, XII, 1-30) [the quote is from pp. 100-101, Borges also mentions it on p. 14]
To be completely honest, I don't see this in Project Gutenberg's translation of the Inferno (below); this page says
A common subject of early Greek art, the Minotaur is usually depicted as an animalistic man with a bull's head and tail; some have argued that Dante intended the features to be reversed, though. [emphasis added; no ref given]
However, if you dig back to a 14th c. Italian manuscript of the Inferno in the Bodleian library at Oxford you do indeed get a picture of a bull with the head [and torso!] of a man ...
Canto XII, 10-25
Such was the passage down the precipice high.
And on the riven gully’s very brow
Lay spread at large the Cretan Infamy*
Which was conceived in the pretended cow.
Us when he saw, he bit himself for rage
Like one whose anger gnaws him through and through.
‘Perhaps thou deemest,’ called to him the Sage,
‘This is the Duke of Athens drawing nigh,
Who war to the death with thee on earth did wage.
Begone, thou brute, for this one passing by
Untutored by thy sister has thee found,
And only comes thy sufferings to spy,’
And as the bull which snaps what held it bound
On being smitten by the fatal blow,
Halts in its course, and reels upon the ground,
The Minotaur I saw reel to and fro;
* The minotaur.