In Latin we have the ablative case. Its common uses can be described as instrumental and locative (ablativus loci). But in Slavonic languages we have a distinct locative case.
Did the instrumental and locative merge into one case in Latin, or did the Slavonic language invent the separate cases? When did this happen? Why don't we have a distinct locative case in Latin?
There is a locative case in Proto-Indo-European, but in many later languages it merged into other cases, Slavonic languages being an exception. (So Slavonic didn't invent the locative case.)
Old Latin had a functioning locative case, but for a number of reasons (like shift in pronunciation), the locative case merged for the most part into the ablative in Classical Latin.
So, in Classical Latin the locative is only fully functioning for small islands and towns or cities (e.g. Roma → Romae, Athenae → Athenis). In addition, there are a few fossilised instances of the locative case, like domī, rūrī, humī. You probably learn these as exceptions in a basic (Classical) Latin textbook.
visam, si domi est
I will see, if he is home
from Heauton Timorumenos
Both Latin and Slavic languages descend from parent Proto-Indo-European language spoken around 3500 BC north of the Caspian Sea. PIE language, as it is reconstructed, had the following cases:
Latin, through its development, merged instrumental, ablative and locative in one case that is traditionally called ablative.
On the other hand Slavic languages preserved them better, losing only ablative.
You might wanto to look into the following publication for more information on that topic:
Indoeuropean Language and Culture, Benjamin Fortson
