Who are these ʻother sheepʻ? The answer is the Gentiles (those not Jewish) We can take a look at some respected Catholic biblical commentaries as well as the writings and homilies of the Early Church Fathers to expound :
MacEvilly, in his commentary states “ ʻAnd other sheep I haveʻ etc. He refers to the Gentile world, whom He meant to call into His Church, bestowing on them the priceless gift of faith" (MacEvilly, J. (1902). An Exposition of the Gospel of St. John (209) Benziger Brothers.
Haydock continues the same sentiment by saying that the signification of the ʻone foldʻ later in the verse is that "there shall be one church of Jews and Gentiles converted." (Haydock's Catholic Bible Commentary, 1859)
St Thomas Aquinas, in his famous Catena Aura, confirms this understanding by showing the commentaries of several Church Fathers including Pope St. Gregory I (A.D. 590) and the Doctor of Grace, St. Augustine
" Gregory. (Hom. xiv.) But as He came to redeem not only the Jews, but the Gentiles, He adds, And other sheep I have, which are not of this fold.
image used through the 'Creative Commons Attribution-Share Alike 3.0 Unported license. via wikimedia commons. Stained glass: Alfred Handel, d. 1946