Not always.
OED:
1.a. A human being much below the ordinary stature or size; a pygmy.
All the quotations for that sense use a singular form except for one
which uses the spelling: "dwerghs".
1.b. One of a supposed race of diminutive beings, who figure in
Teutonic and esp. Scandinavian mythology and folk-lore; often
identified with the elves, and supposed to be endowed with special
skill in working metals, etc.
1770 T. Percy tr. P. H. Mallet Northern Antiq. I. vi. 106 They
made of his scull the vault of heaven, which is supported by four
dwarfs named South, North, East and West.
1818 W. Taylor in Monthly Mag. 46 26 The history of Laurin, king
of the dwarves.
1834 E. Bulwer-Lytton Pilgrims of Rhine xxvi, The aged King of
the Dwarfs that preside over the dull realms of lead.
1846 J. E. Taylor Fairy Ring Notes 363 The notion that the
wicked elves or dwarfs had the power to steal children before
their baptism is found also..in Iceland.
2.a. An animal or plant much below the ordinary height or size of
its kind or species.
1691 J. Evelyn Kalendarium Hortense (ed. 8) 129 The Crab-stock
for Standards: For Dwarfs, Stocks of the Paradise or sweet
Apple-kernel.
1719 G. London & H. Wise J. de la Quintinie's Compl. Gard'ner (ed.
7) 113 The Beauty of Dwarfs consists in a low Stem, an open
Head.
Spellings through the ages:
Forms: Pl. dwarfs; Forms: a. OE duerg, dweorg, dweorh, ME dwæruh,
ME dweru?, ME dwer?(e, ME dwergh, dwargh(e, duergh, dwerk, ME–15 Sc.
duerch(e, dorche, droich n. ß. ME dweruf, ME dwerf(e, dwerff(e,
(dwrfe), ME–16 dwarfe, 15–16 dwarff(e, 16 dwearf, ME– dwarf. ?. ME
duerwe, durwe, dwarw, ME dwerwh(e, dwerwe, dwerowe, duorow. d. ME
dwery, duery, dueri.