"Words neuter in Latin become masculine in Spanish"
This is generally correct!
In Latin, the most common type of masculine noun and the most common type of neuter noun look almost identical. They're only distinguishable in the nominative singular and the nominative/accusative plural.
In the branch of Romance that would eventually become Spanish, the ...
This is just to provide evidence for cōs-cōtis from classical poets:
saepe etiam duris errando in cotibus alas
(Vergilius, Georgica, 4.203)
(Horatius, Carmina 2.8.16)
nil tanti est. Ergo fungar vice cotis, acutum
(Horatius, Ars Poetica, 304)
All of these scan right if and only if cot- is read with a long vowel.
These are all the examples of ...