Masculine and feminine (or common) nouns in modern Scandinavian languages typically have the formative -Vr in the (nominative) plural:

(1)	Icelandic: fisk-ar ‘fish.M-PL.NOM’
	Swedish: fisk-ar ‘fish.C-PL’
	Faroese: fisk-ar ‘fish.M-PL.NOM’
(2)	Icelandic: geit-ur ‘goat.F-PL.NOM’
	Swedish: gett-er ‘goat.C-PL’
	Swedish: geit-ir ‘goat.F-PL.NOM’

Modern German has a range of masculine and neuter nouns that take -er in the plural. There are far more such nouns in the modern language than in Old / Middle High German:

(3)	German: Männ-er ‘man.M-PL’
	German: Nest-er ‘nest.N-PL’

A lot of Germanic language enthusiasts take these to be the same historically. This short post shows that they are not.

The Scandinavian (nominative) plural suffixes -or/-ur/-ar/-ir/-er continue the non-neuter Indo-European -Vs nominative plural formative, with the vowel quality varying depending on inflectional class features. The consonant /s/ in these suffixes was at some point voiced in Proto-Germanic, yielding -Vz. Finally, the process of rotacism (a shift of a voiced coronal fricative to a trill or, more likely, tap) yielded a conversion of the voiced final /z/ to /r/ (as it was also the case in other environments):

(4)	fisk-as/-os > fisk-az > fisk-ar

The German equivalent of these inflectional classes is actually the plural with -e, with or without umlaut. The reason for this is that the consonant /z/ was eliminated word-finally in many suffixes during the evolution of German.

(5)	German: stain-az > stein-a > Stein-e ‘stone.M-PL’
	Icelandic: stain-az > stein-ar ‘stone.M-PL.NOM’
	English: stain-az > stān-as > stān-es > stone-s

The loss of the final -s/-z was common in German, but not in English, Dutch and most Scandinavian varieties. However, notably, some common (formerly masculine and feminine) nouns in Danish have also dropped the final -r: dreng-e, hund-e, søstr-e (cf. by-er). These Danish plurals match the German ones in their evolution.

The -Vr of Scandinavian thus directly corresponds to the normal plural formative -(e)s in Modern English (which has been extended to nearly all nouns by analogy).

As far as the German suffix -er is concerned, it was originally restricted to a handful of nouns belonging to the so-called athematic declension in the old Indo-European languages. These nouns had consonantal, rather than vocalic, stem formatives, usually -Vr-, -Vs-, and -Vn-.

(5)	Old German: lamb ‘lamb’ > lemb-ir-un ‘lamb-R-PL.DAT’
cf.	Lithuanian: dukt-ė ‘daughter’ > dukt-er-ys ‘daughter-R-PL.NOM’
cf.	Old German: nam-o ‘name’ > nam-ōn-o ‘name-N-PL.Gen’
cf.	Russian: neb-o ‘sky’ > neb-es-a ‘sky-S-PL.NOM’

Originally, the stem-forming suffixes were restricted to a handful of athematic nouns. In Modern German, however, the original stem formative -on- has become: (a) the marker of oblique singular cases for all weak masculines, many formed by analogy with original athematic nouns (dem Herr-n, Schwede-n, Hase-n etc.); (b) the plural marker for all weak masculines, some neuters and some 90% of all feminine nouns, most of which was, once again, done by analogy because most feminines were thematic in Old German (Schwester-n, Bank-en, Amsel-n, Uhr-en etc).

The stem extendor -ir- was originally employed by a small number of neuter nouns. The fact that the vowel in it was the front /i/ explains why this formative consistently triggers umlaut in Modern German, even though the vowel has since been reduced to a shwa, making the process opaque.

With most of the case endings having been lost, the -ir- stem formative was reanalyzed as a plural marker (just like the original -on- discussed above) and its area of application extended to many originally thematic neuter nouns. Even some masculine nouns (originally thematic and athematic) have shifted into this class.

(6)	Lamm > Lämm-er ‘lamb.N-PL’ (originally ‘lemb-ir-Ø’, R historically motivated)
	Gesicht > Gesicht-er ‘face.N-PL’ (originally ‘gisiht-Ø’, R by analogy!)
	Mann > Männ-er ‘man.M(!)-PL’ (originally ‘mann-a’, R by analogy!)
	Wurm > Würm-er ‘worm.M(!)-PL’ (originally ‘wurm-a’, R by analogy!)

In (6), only the first one of the four nouns taking -er in the plural was originally a true R-noun akin to Modern Lithuanian ‘dukt-er- (daughter)’ or Slavic ‘mat-er- (mother)’. The other three made it into this inflectional class by analogy, with two of them being exceptionally masculine.

All in all, the Scandinavian -Vr plural regularly corresponds to the German plural in -e (<< -Vz) and the English plural in -(e)s. Thus, while it has been innovating in dropping all case endings, the English plural is the most ‘original-looking’ of all. The German plural regularly lost the final coronal fricative: this can also be seen in one of the inflectional classes of Modern Danish. The pervasiveness of the suffix -er in contemporary German plurals is due to the expansion of an originally small class of athematic nouns inherited from Proto-Indo-European. Interestingly, the R-class nouns have mostly lost this property in Modern Scandinavian:

(7)	Swedish: lamm > lamm-Ø (PL)