Apparemment on peut utiliser uruk pour le singulier et uruks pour le pluriel, mais évidemment ça doit être le pluriel anglais uniquement... Sur Ardalambion, on trouve l'hypothèse que -hai signifie "race" en Noir Parler. Cette hypothèse semble vérifiée notamment par cette phrase des Contes et Légendes inachevés:
"These figures served not merely as insults to their enemies; for the Orcs feared them and believed them to be filled with the malice of the Oghor-hai (for so they named the Drúedain), and able to hold communication with them."
Finalement il y a une phrase qui prouve je crois que l'Uruk-hai de Saroumane est constituée d'uruks normaux:
"Orcs began to raid in the eastern regions and slay or steal horses. Others also came down from the Misty Mountains, many being great uruks in the service of Saruman, though it was long before that was suspected." (voir ci-dessous pour la réf.)
Il reste que je n'ai pas trouvé de passage où Uruk-hai s'applique aux orques de Sauron, mais c'est probablement un hasard...
Si quelqu'un trouve une citation plus explicite pour prouver définitivement que les deux termes désignent à la fois les orques de Sauron et de Saroumane...
En attendant, ma question devient: quand voit-on les "nouveaux orques" de Saroumane?
Pour ceux que ça intéresse, voilà toutes les références que j'ai trouvées pour uruk et uruk-hai dans le SDA:
Gandalf dans la Moria: "black Uruks of Mordor"
Dans le chapitre "The Land of Shadow":
"Beside them, running up and down the line, went two of the large fierce uruks, cracking lashes and shouting."
"A troop of heavy-armed uruks from Barad-dûr"
(en italique dans le texte)
Dans The Choices of Master Samwise:
"`Yes,' said Gorbag. 'But don't count on it. I'm not easy in my mind. As I said, the Big Bosses, ay,' his voice sank almost to a whisper, `ay, even the Biggest, can make mistakes. Something nearly slipped you say. I say, something has slipped. And we've got to look out. Always the poor Uruks to put slips right, and small thanks."
Dans l'appendice A, The Stewards:
"In the last years of Denethor I the race of uruks, black orcs of great strength, first appeared out of Mordor, and in 2475 they swept across Ithilien and took Osgiliath."
Appendice A, The Kings of the Mark:
"Orcs began to raid in the eastern regions and slay or steal horses. Others also came down from the Misty Mountains, many being great uruks in the service of Saruman, though it was long before that was suspected."
Appendice B:
2901 Most of the remaining inhabitants of Ithilien desert it owing to the attacks of Uruks of Mordor. The secret refuge of Henneth Annûn is built.
Pour uruk et uruk-hai:
Appendice F, Of other Races:
"Orcs and the Black Speech. Orc is the form of the name that other races had for this foul people as it was in the language of Rohan. In Sindarin it was orch. Related, no doubt, was the word uruk of the Black Speech, though this was applied as a rule only to the great soldier-orcs that at this time issued from Mordor and Isengard. The lesser kinds were called, especially by the Uruk-hai, snaga 'slave'."
Pour uruk-hai:
Dans le chapitre "the Uruk-hai":
"We are the fighting Uruk-hai!" revient à plusieurs reprises (à chaque fois ce sont les orques de Saroumane qui le disent)
"'Very well,' said Uglúk. 'Leave them to me then! No killing, as I've told you before; but if you want to throw away what we've come all the way to get, throw it away! I'll look after it. Let the fighting Uruk-hai do the work, as usual."
"'You seem to know a lot,' said Uglúk. 'More than is good for you, I guess. Perhaps those in Lugbúrz might wonder how, and why. But in the meantime the Uruk-hai of Isengard can do the dirty work, as usual."
Dans The Siege of Gondor:
"No hours so dark had Pippin known, not even in the clutches of the Uruk-hai."
The Land of Shadow:
"'Whose blame's that?' said the soldier. 'Not mine. That comes from Higher Up. First they say it's a great Elf in bright armour, then it's a sort of small dwarf-man, then it must be a pack of rebel Uruk-hai; or maybe it's all the lot together.'"