Although the papal ban seems to have greatly reduced the consumption of horseflesh in most of Europe, the ritual sacrifice of horses continued for a surprisingly long time. Horses were slaughtered at the funerals of King John of England in 1216 and the Holy Roman Emperor Karl IV in 1378. As recently as 1781, during the funeral of cavalry General Friedrich Kasimir at Trier, his horse was killed and deposited in his grave.
The source for this claim, which is widespread online, appears to be A History of Pagan Europe by Prudence Jones and Nigel Pennick (Routledge, 1995), p. 140:
Pagan horse sacrifice continued in Denmark until the early eleventh century [footnote 6], and it continued as a funeral rite of kings and knights. Horses were slaughtered at the funerals of King John of England [footnote 7], the Emperor Karl IV in 1378 and Bertrand Duguesclin in 1389.[footnote 8] In 1499, the Landsknechte sacrificed a horse to celebrate the end of the Swabian Wars (Schwabenkrieg).[footnote 9] During the funeral of Cavalry General Friedrich Kasimir at Trier in the Rhineland in 1781, his horse was killed and thrown into his grave.[footnote 10]
It's published by a reputable if non-university publisher. It has a nice font. It has footnotes. And there is a long history of pagan survivals in Europe; the 1781 incident sounds plausible. So you'd think the book is reliable.
But the notion of horses being slaughtered over King John's grave, in full view of the ecclesiastical hierarchy of England, just doesn't pass the smell test.
Reason (a) to be wary of the claim: reviews of the book at Goodreads:
I'm always a little leery of historians with an obvious axe to grind, and the authors of this book - better known for a series of New Age and Wiccan publications - definitely qualify
I'll also say that the bibliography is helpful, and there are a fair number of footnotes (though still, not enough), which is unusual for something Nigel Pennick had a hand in. (In his other books you're always thinking, "Wow, that's interesting. I'd like to know more, but hmmm...there's no reference. How does he know that??")
Reason (b), no independent mention of this in the Wikipedia page on King John, the PhD thesis on Burials of English Kings (that starts with King John), or the French Wikipedia page on du Guesclin (which spends six paragraphs on his quadruple burial). You'd have thought it a noteworthy enough detail to mention.
Reason (c): Google Books does not show Jones & Pennick's footnotes; but I think I did find their source, Tylor's Primitive Culture (1873) (p. 474), and that's not what Tylor said:
All these rites probably belong together as connected with ancient funeral sacrifice, and the survival of the custom of sacrificing the warrior's horse at his tomb is yet more striking. Saint-Foix long ago put the French evidence very forcibly. Mentioning the horse led at the funeral of Charles VI., with the four valets-de-pied in black, and bareheaded, holding the corners of its caparison, he recalls the horses and servants killed and buried with præ-Christian kings. And that his readers may not think this an extraordinary idea, he brings forward the records of property and horses being presented at the offertory in Paris, in 1329, of Edward III. presenting horses at King John's funeral in London, and of the funeral service for Bertrand Duguesclin, at St. Denis, in 1389, when horses were offered, the Bishop of Auxerre laid his hand on their heads, and they were afterwards compounded for. [Saint-Foix, "Essais historiques sur Paris", in "Oeuvres Comp." Maestricht, 1778, vol. iv. p. 150.]
Germany retained the actual sacrifice within the memory of living men. A cavalry general, Count Friedrich Kasimir Boos von Waldeck, was buried at Treves in 1781 according to the forms of the Teutonic Order; his horse was led in the procession, and the coffin having been lowered into the grave the horse was killed and thrown in upon it. (J. M. Kemble, "Horae Ferales", p. 66.) This was, perhaps, the last occasion when such a sacrifice was consummated in solemn form in Europe. But that pathetic incident of a soldier's funeral, the leading of the saddled and bridled charger in the mournful procession, keeps up to this day a lingering reminiscence of the grim religious rite now passed away.
(The reference Tylor cites for von Waldeck does not give the details he does.)
That's why there was no mention of any of this in the PhD thesis about the burial of English Kings, by the way: as @MAGolding point out, the burial was not of King John of England, who lived a century and a half before King Edward III, but of John II of France, who died in London in 1364 as Edward's prisoner. Edward III was kind enough to send the horses with the body back to Paris for burial.
Some new references may have been unearthed by Jones & Pennick, and I have not seen their footnotes; user @justCal did find the source they gave for their footnote 7 (King John), and it has if anything less information than Tylor (and certainly less than Saint-Foix): Johann Nepomuk Sepp, Die Religion der alten Deutschen und ihr Fortbestand in Volkssagen, Aufzügen und Festgebrauchen (1890), p. 267. (My translation from German):
Eduard III brought horses to the Offertory at King John's funeral in London. The Bishop of Auxerre blessed the horses in 1389, which were offered (geopfert) to St Denys at the funeral service of Bertrand Duguesclin.
Sepp cites Tylor for von Waldeck, in fact. And (linked above) this is what Saint-Foix had written (again, my translation):
In a transaction dated 1329 between the priests of Paris and the Church of the Holy Sepulchre, it was written that someone dying was free to choose to be buried in that church, but that his body would first be brought to the parish where he died, and that the priests of that parish would have half the lamps, herds and horses to be offered up (présentés a l'offrande), before he was buried in the Holy Sepulchre. The Continuator of Nangis reports that when King John died in London, Edward III carried out a magnificent service for him, and that he offered up several prize horses, caparisoned in black, with the French escutcheon ("offerens pro eo multos equos insignitos armis Franciae, cum equitibus..."). In the service in 1389 at Saint Denis for Bertrand Duguesclin, by the order of Charles VI [...] [citing Hist. de l'Abbaye de S. Denis, D. Félibien] "The Bishop received the present of horses by placing his hand on their head; then they were brought back (on les ramena): but they had to be later compensated for with the abbey, in which they were vested."
I have reason to assume Jones & Pennick have misread "offering" (German: geopfert) horses in their sources with regard to Christian royal burials, as literal sacrificial killing rather than the attenuated Christian notion of blessing at the altar. (The German verb is translated as both "sacrifice" and "offer up". In Saint-Foix's account, lamps are being offered up in the same breath as sheep and horses—they are hardly being slaughtered. Not to mention that the Latin chronicle speaks of offering up horses and their knights.) The sources also sound to me like the churches involved got to keep the horses (alive) after they were offered up.
So much for the evidence of the 14th century. Does anyone have any further evidence for or against a survival of horse sacrifice in Christian mediaeval Europe?