In Theological Writings:

St. Gregory the Great (6^th century):

"Indeed the horse is the body of any holy soul, which it knows how to restrain from illicit action with the bridle of continence and to release in the exercise of good works with the spur of charity."

- "Equus quippe est unicuique sanctae animae corpus suum, quod videlicet novit et ab illicitis continentiae freno restringere, et rursum charitatis impulsu in exercitatione boni operis relaxare." - Commentary on Chapter 25, verse 18 of the Book of Job.

- Patrologia Latina (ed. Migne) vol.76, col. 588 [From Robertson, A Preface to Chaucer p. 254.]

Commentary on the Book of Proverbs, attributed to Robert Holcot (14^th century British friar and scholar):

"Thus moraliter our flesh is the horse and the reason spirit is the rider."

"Sic moraliter caro nostra equus est et ratio sive spiritus est sessor."

(See an image of this very sentence from the 1510 edition of Holcot's work by clicking on this image -> img3.jpg )

- https://www.google.com/books/edition/In_proverbia_Salomonis_Explanationes/nzk8AAAAcAAJ?hl=en&gbpv=1&dq=%22In+proverbia+Salomonis%22+Holcot&pg=PP26&printsec=frontcover

- In proverbia Salomonis (Paris, 1515), fol. ix. [From Robertson, A Preface to Chaucer p. 254.]

St. Augustine (4^th-5^th century):

"Let us subjugate the flattery or troublesomeness of this cupidity. Let us subjugate to us this woman, if we are men."

"Vincamus ergo huius cupiditatis uel blanditias uel molestias. Subiugemus nobis hanc feminam, si uiri sumus."

- De vera religione, 41.78 [After Robertson, A Preface to Chaucer p. 254.]

"de clerico et puella" (Middle English, 15^th century):

Yef thou in my boure art take,
Shame thee may bityde.
Thee is bettere on fote gon
Then wycked hors to ryde.

- http://www.outlawsandhighwaymen.com/clerico.htm

Mankind: (English, 15^th century):

Mercy:

241 Yf a man haue an hors and kepe hym not to hye,
242 He may then reull hym at hys own dysyere.
243 Yf he be fede ouerwell he wyll dysobey
244 Ande in happe cast his master in þe myre.

245 NEW GYSE. 3e sey trew, ser, 3e are no faytour.
246 I haue fede my wyff so well tyll sche ys my master.

- https://web.archive.org/web/20160724104710/http://research.uvu.edu/mcdonald/3610/mankind.html

Chaucer, Prolog to the Canterbury Tales: (from http://etext.lib.virginia.edu)

165: A monk ther was, a fair for the maistrie,
166: An outridere, that lovede venerie,
167: A manly man, to been an abbot able.
168: Ful many a deyntee hors hadde he in stable…

189: Therfore he was a prikasour aright:
190: Grehoundes he hadde as swift as fowel in flight;
191: Of prikyng and of huntyng for the hare
192: Was al his lust, for no cost wolde he spare.