Origionally, rainbow trout were called: Salmo iridia due to the colouration.
The modern name Oncorhynchus mykiss
is Greek, not Latin.
It means "hooknose", which is so typical of a male rainbow. Mykiss refers to an area of North America where the species is endemic.
No doubt the scientists will change "proper" names when they feel like it. Several African species of Barbus have changed the proper name quite recently.
In scientific circles by the way, the scientific name is called the "proper" name. All other names are called "common".