See also the Dog Collar Museum at Leeds Castle, Mediaeval Collars for the Modern Dog, as well as these links.

• Annunciation, Gospel Lectionary (PML G.44, fol. 2v), c. 1050
• Scenes from the Life of Joachim: Joachim among the Shepherds by Giotto di Bondone, 1304-1306
• A man with a pair of dogs, The Luttrell Psalter (British Library MS Add. 42130, fol. 64v), c. 1325-1335
• The Romance of Alexander (Bodl. 264), 1338-1344: Several pictures of dogs throughout the manuscript, but fol. 77v has an especially useful picture of leashes
• The Book of Modus and Ratio (BNF Fr. 12399), 1379: Hunting a hind (fol. 18v), Hunting a boar (fol. 23), Hunting a boar (fol. 23v), and Hunting a hare (fol. 43v)
• Leaded glass: a dog, from a Dominican cloister in Ghent, 14th-15th century
• Textile fragment with a pattern of dogs, Brussels, 14th-15th century

Twey whyte grehoundys ronne hyr by - Har colers were of golde. Sir Launfal, ll. 965-966, late 14th century

• Several illustrations from the hunting-book of Gaston Phoebus; this link in particular goes to BNF Fr. 616 (beginning of the 15th century), but you can also use Mandragore for BNF Fr. 1291 (c. 1445-1450) and BNF Fr. 617 (beginning of the 16th century), or see PML M.1044, fol. 45 (c. 1410).
• Parsley (fol. 31v), mandrake (fol. 37), gazelle (fol. 69v), hare (fol. 70), and hunting (fol. 93), Tacuinum Sanitatis (BNF Latin 9333), 15th century
• Tristan and the shepherds, Tristan de Léonois (BNF Fr. 97, fol. 136v), 1st quarter of the 15th century
• Charles VI talking to Pierre Salmon, 1412
• January and December from the Tres Riches Heures du Duc de Berry, c. 1412-1416
• The Adoration of the Magi by Gentile da Fabriano, 1423 (part of it is easier to see in this detail)
• Detail from a bas-relief of the story of Esau and Jacob, by Lorenzo Ghiberti, 1444-1452
• Several in a manuscript of the Book of the Hunt of Gaston Phoebus c. 1445-1450 (BNF Fr. 1291), including fols. 26v and 34v
• Presentation scene in the Chronicle of Hainaut, 1447-1448
• A dog in a bestiary (MMW 10 B 25, fol. 15v), c. 1450
• A woman and a dog supporting a heraldic escutcheon (Getty 7, fol. 2v) from the Book of Hours of Simon de Varie; the dog's collar has Varie's motto
• Philosophy enthroned, The City of God (BNF Fr. 27, fol. 232), 3rd quarter of the 15th century
• Massinissa guarded by his dogs, Facta et dicta memorabilia (BNF Fr. 289, fol. 483v), second half of the 15th century
• The herald and his cat (an ocelot? a cerval? a jaguar?) in the Procession of the Old King in the Procession of the Magi frescoes by Benozzo Gozzoli, 1459-1460
• The feast of King Louis le Gros in The Romance of Girart de Nevers (fol. 2v), c. 1460
• Esau gives up his birthright, Speculum Historiale (BNF 50, fol. 45v), 1463
• Clovis I commands the exhumation of Apollo, Tristan de Léonois (BNF Fr. 99, fol. 30), 1463
• Annunciation (note spiked collar), a book of hours (PML M.1003, fol. 82v), c. 1465
• Lancelot du Lac (BNF Fr. 112(1)), c. 1470: Lancelot beats his master (fol. 4v) and Saraide deceives Claudas (fol. 5v)
• Alboin and Rosamund feasting and the death of Alboin, Confessio Amantis (PML M.126, fol. 23r), c. 1470
• Fol. 78r in the Chronicle of Baudouin d'Avesnes, c. 1470-1488
• Detail from the frescoes in the suite of Cardinal Francesco at the Palazzo Ducale in Mantua by Andrea Mantegna, 1471-1474
• Triptych with Jean de Witte and Maria Hoore (detail), 1473
• Flavius Josephus in his study, Antiquities of the Jews (BNF Fr. 11, fol. 1), fourth quarter of the 15th century
• The Wayfarer by Hieronymus Bosch, c. 1480
• Two white greyhounds in the Tournament-book of King René d'Anjou (BNF Fr. 2692, fol. 9v), c. 1488-1490
• January and August in the Grimani Breviary, c. 1490-1510
• Diptych with Jean de le Cambe and St. John the Baptist, c. 1490-1510
• The Unicorn Tapestries, 1495-1505
• The Garden of Pleasure, Échecs amoureux (BNF Fr. 143, fol. 198v), c. 1496-1498
• Detail from The Judgment of Cambyses by Gerard David, 1498
• Diana and Évrard de conty, Échecs amoureux, end of the 15th century
• Semiramis, De mulieribus claris (BNF Fr. 599, fol. 5v), 15th or 16th century
• Tile (from the palace of Isabella d'Este) with the emblem of Gianfranco II de Gonzaga, c. 1500
• Detail from St. Wolfgang Enters Regensburg, c. 1500-1510
• Phaedra departs for the hunt, Ovid's Heroides (BNF Fr. 874, fol. 31v-32), beginning of the 16th century
• Faience albarello with a cherub and a dog, c. 1500-1520
• Two Venetian Ladies by Vittore Carpaccio, c. 1510
• King Solomon accepts gifts from the Queen of Sheba, 1520
• Two shepherds, early 16th century
• Fols. 2r, 4v, and 12v in the Hennessy Book of Hours, c. 1530-1540
• Portrait of Charles V by Jacob Seisenegger, 1532
• Portrait of Charles V by Titian, 1532-1533
• Hunting near Hartenfels Castle by Lucas Cranach, 1540
• Portrait of Jacquemyne Buuck (detail) by Pieter Pourbus, 1551
• Several illustrations in La Venerie by Jaques du Fouiloux, 1561
• Diana the Huntress, second half of the 16th century
• Studies of a Greyhound by Federico Zuccaro, 1563-1564
• Fête at Bermondsey by Joris Hoefnagel, 1569
• Of Our English Dogs And Their Qualities from Harrison's Description of England, 1577
  Doesn't actually have much to do with leashes & collars, but it's an interesting bit to read.
• Labors of the Months: November, c. 1580