"The Panther" is an Old English poem, preserved in the Exeter Book, and translated in full by Aaron Hostetter. It's a poem about a panther, but what would the ancient Germanic peoples of Europe have understood by "panther"? Nowadays, the word panther can be used to mean any of three cat species, two found only in the Americas, the third being the leopard. Would the Anglo-Saxons have known about leopards? The Romans did, but then the Romans had an empire which stretched as far as Asia Minor and North Africa, and I don't know how much of their menagerical knowledge survived in northwestern Europe after the collapse of their empire.
The OED has a note:
panther, n. 1.a. Originally: an exotic spotted big cat that was believed to be distinct from the leopard.
As with other exotic animals, the name, handed down from the Latin writers, was known long before the animal; all the early references merely reflect the statements of ancient authors and their medieval continuators. These statements were long believed to refer to a beast distinct from the leopard. This belief was encouraged by the existence of two Latin names, panthēra and pardus; the ancient writers themselves were not clear as to the relation between the two. The perceived distinction was further perpetuated by fabulous notions regarding the generation of the leopard as a hybrid between the lion and the ‘pard’, and also regarding the sweet fragrance fabled to be exhaled by the panther.
Oxford English Dictionary.
Pliny was popular in medieval Europe and his description of the panther is a likely Latin source for some of the elements of the Old English poem:
panthera et tigris macularum varietate prope solae bestiarum spectantur, ceteris unus ac suus cuique generi color est, leonum tantum in syria niger. pantheris in candido breves macularum oculi. ferunt odore earum mire sollicitari quadripedes cunctas, sed capitis torvitate terreri; quam ob rem occultato eo reliqua dulcedine invitatas corripiunt. sunt qui tradant in armo iis similem lunae esse maculam crescentem in orbem seque cavantem pari modo. nunc varias et pardos, qua mares sunt, appellant in eo omni genere, creberrimo in africa syriaque. quidam ab his pantheras candore solo discernunt, nec adhuc aliam differentiam inveni.
The panther and the tiger are nearly the only animals that are remarkable for a skin distinguished by the variety of its spots; whereas others have them of a single colour, appropriate to each species. The lions of Syria alone are black. The spots of the panther are like small eyes, upon a white ground. It is said that all quadrupeds are attracted in a most wonderful manner by their odour, while they are terrified by the fierceness of their aspect; for which reason the creature conceals its head, and then seizes upon the animals that are attracted to it by the sweetness of the odour. It is said by some, that the panther has, on the shoulder, a spot which bears the form of the moon; and that, like it, it regularly increases to full, and then diminishes to a crescent. At present, we apply the general names of varia and pard, (which last belongs to the males), to all the numerous species of this animal, which is very common in Africa and Syria. Some writers distinguish the panther, as being remarkable for its whiteness: but as yet I have not observed any other difference between them.
Pliny the Elder (c. 77). Natural History, book 8, chapter 23 Translated by John Bostock and H. T. Riley (1855), p. 274. London: Henry G. Bohn.
Pliny’s claim about the panther attracting prey by its scent also appears in Aristotle:
They say that the panther is aware that its peculiar scent is grateful to other wild animals, and that it preys upon them in concealment, and when deer approach near, it catches hinds.
Aristotle (4th century BCE). History of Animals, book 9, chapter 7. Translated by Richard Cresswell (1897), p. 238. London: George Bell.
The “fabulous notions regarding the generation of the leopard as a hybrid between the lion and the ‘pard’” are also found in Pliny:
leoni praecipua generositas tunc, cum colla armosque vestiunt iubae; id enim aetate contingit e leone conceptis. quos vero pardi generavere, semper insigni hoc carent; simili modo feminae. magna his libido coitus et ob hoc maribus ira. africa haec maxime spectat, inopia aquarum ad paucos amnes congregantibus se feris. ideo multiformes ibi animalium partus, varie feminis cuiusque generis mares aut vi aut voluptate miscente: unde etiam vulgare graeciae dictum semper aliquid novi africam adferre. odore pardi coitum sentit in adultera leo totaque vi consurgit in poenam; idcirco ea culpa flumine abluitur aut longius comitatur.
The noble appearance of the lion is more especially to be seen in that species which has the neck and shoulders covered with a mane, which is always acquired at the proper age by those produced from a lion; while, on the other hand, those that are the offspring of the pard, are always without this distinction. The female also has no mane. The sexual passions of these animals are very violent, and render the male quite furious. This is especially the case in Africa, where, in consequence of the great scarcity of water, the wild beasts assemble in great numbers on the banks of a few rivers. This is also the reason why so many curious varieties of animals are produced there, the males and females of various species coupling promiscuously with each other. Hence arose the saying, which was common in Greece even, that “Africa is always producing something new.” The lion recognizes, by the peculiar odour of the pard, when the lioness has been unfaithful to him, and avenges himself with the greatest fury. Hence it is, that the female, when she has been guilty of a lapse, washes herself, or else follows the lion at a considerable distance.
The answer to your question about the use of “Leopards and “Panthers” in this case has little to do with real animals but is concerned instead with old religious symbolism. This other information can be found in most ‘Bestiaries.’ First of all, in these old books they made very little distinction between panthers and leopards. Here is a short quotation - under Panther - from T. H. White’s translation of one of these “instructive” books:
There is an animal called a PANTHER which has a truly variegated colour and is most beautiful, and excessively kind. Physiologus says that the only animal which it considers as an enemy is the dragon. (p. 14).
From here the reader is further informed that the Panther stands for Jesus and the dragon for the Devil, the three days spent in the “cave” are the days Jesus spent harrowing hell, while the sweet scent that attracts the other animals means the “Good Word,” etc. With the above information one can now go back to ‘The Panther’ and much better understand the nature of this allegorical poem, and the place assigned to the Panther.