In Brazilian Portuguese an acoustic guitar is called violão, while the electric guitar is called guitarra. The Portuguese guitar is a very different instrument. The rest of the world calls it a Portuguese guitar, but in Portugal it’s simply a guitar. So a different word was used for what the Spanish call a guitar. 

What I find interesting is that the guitar is female in all romance languages and even in German, which tends to try to be different. (the moon is male in German and the sun female. Craziness!!) But while it’s a guitarra (female) it is o violão (male). I supposed this shows that all languages develop around what feels good saying, not around what actually makes sense – as shown by aluminum or critter in American English – and perhaps people found that o violão and a guitarra sounded better. Or perhaps these genders really don’t mean anything… they are simply a way to make language more complicated. It’s something that English has solved beautifully.