Allen and Greenough's New Latin Grammar, section 106

Heavy Construction

The Allen and Greenough is still under construction; so some links may not work quite the way you would expect.

106.

Nouns that vary in Gender are said to be heterogeneous.[1][That is, ``of different genders'' (o , another, and o , gender).]

a. The following have a masculine form in -us and a neuter in -um: balteus, cáseus, clipeus, collum, cingulum, píleus, tergum, vállum, with many others of rare occurrence.

b. The following have in the Plural a different gender from that of the Singular: -

balneum (N.), bath; balneae (F.), baths (an establishment).
caelum (N.), heaven; caelós (M. acc., Lucr.).
carbasus (F.), a sail; carbasa (N.) (-=orum), sails.
délicium (N.), pleasure; déliciae (F.), pet.
epulum (N.), feast; epulae (F.), feast.
frénum (N.), a bit; fréní (M.) or fréna (N.), a bridle.
iocus (M.), a jest; ioca (N.), iocí (M.), jests.
locus (M.), place; loca (N.), loc=i (M., usually topics, passages in books)
rástrum (N.), a rake; rástri (M.), rástra (N.), rakes.

NOTE: Some of these nouns are heteroclites as well as heterogeneous.

balneum (N.), bath;	balneae (F.), baths (an establishment).
caelum (N.), heaven;	caelós (M. acc., Lucr.).
carbasus (F.), a sail;	carbasa (N.) (-=orum), sails.
délicium (N.), pleasure;	déliciae (F.), pet.
epulum (N.), feast;	epulae (F.), feast.
frénum (N.), a bit;	fréní (M.) or fréna (N.), a bridle.
iocus (M.), a jest;	ioca (N.), iocí (M.), jests.
locus (M.), place;	loca (N.), loc=i (M., usually topics, passages in books)
rástrum (N.), a rake;	rástri (M.), rástra (N.), rakes.