So, unfortunately, I stated earlier that the neuter i-stem class is entirely regular. In fact, a footnote in A&G reveals about ten exceptions.
These nouns are almost regular, except that with consonantal stems –al and –ar they also add the ending -e to the nominative and accusative singular. Note that because of this ending, the -ā- is long in all cases.
Where the singular is uncommon or does not exist, I have used the plural.
- alveāre, alveāris, beehive
- augurāle, augurālis, augur’s staff
- capillāre, capillāris, pomade
- cochlearē, cochleāris, spoon
- collāre, collāris, collar
- dentālia, dentālium, sharebeam of a plow (What?)
- fōcāle, fōcālis, cravat (What?)
- nāvāle, nāvālis, dock
- penetrāle, penetrālis, inner shrine
- rāmālia, rāmālium, twigs
- scūtāle, scūtālis, thong of a sling
- tibiālia, tibiālia, shin-length stockings
The Essential AG: 68n2
2 things. First, this category may be at least partly explained by adjectives that became substantized. For example, the noun collus, colli means neck. Then an adjective was formed: collaris, collaris, collare – pertaining to the neck. From the neuter, collare, we get a substantive “thing pertaining to the neck, a collar.” So, this irregular -e ending is really just showing its roots as a neuter adjective. (It’s useful to remember that 3rd declension regular adjectives are i-stems.)
Second, in your second paragraph I think you meant to write, add the ending -e to the nominative and accusative SINGULAR.