Some nouns I have encountered were in the accusative singular and ended in -us. I did find much information on such ending. And from what I gather it happens with 2nd( like Vulgus) and 4th (like artus) declension nouns. Is there any list of nouns with which something like this happens?
Vulgus is a neuter noun and so -us is both the nominative and accusative singualr. (it can also be -um , which presumably means it can be masculine)
Fourth declension nouns ending in -us are -um in accusative sing but -ūs in accusative plural ( and in the genitive singular).
Fourth declension nouns are described here: https://dcc.dickinson.edu/grammar/latin/4th-declension-stem-paradigm-and-gender
virus is another neuter singular noun ending in -us. Also pelagus.
artus is masculine but very rarely used in the singular.