I know that conmigo is standard grammar, as prescribed by RAE. However, on a couple of occasions I've also seen people using con yo. If that helps, I saw that in a Spanish language chatroom where a lot of colloquialism was being thrown around. My question is, is it just plain wrong colloquialism? Or is it possible that con yo is actually the preferred version in some dialect of Spanish somewhere? If so, which dialect would that be?
My vote is for "plain wrong".
I don't think is a colloquialism, nor that a Spanish dialect would admit it, much less as a preferred version. It is just that some people, even native speakers, make some mistakes when speaking.
My 5 year old nephew for example used to say "more good" and "more bad", and the adults would teach the proper ways ("better" and "worse"). I have heard similar things from adults, and this may not mean that the are uneducated or illiterate. Sometimes we just don't realize it when we talk, or you kind of change your mind in mid-sentence, and you end up saying one of these.
Same applied to Spaniards. I remember one of my teachers saying
A quién no se le ha escapado alguna vez un "haiga" (wrong conjugation of "Haber" in presente subjuntivo "haiga" instead of "haya")
I have heard "con yo" (when should have been conmigo), "andé" (when should have been anduve), "conducí" (when should have been conduje), etc. Probably I have said them too, without even realizing and without people correcting me.
So, either people in the chatroom were doing it on purpose to sound funny, or they were using poor Spanish. Honest mistake, I'm sure, but can't be considered a colloquialism and it goes too much against grammar rules to be considered the preferred or accepted form in a Spanish dialect.
It is just a way of joking around. It is wrong and people knows that but you say it to your friends to sound funny or closer and make emphasis in what you want to say. You can say it in relaxed moments, or whenever you want, but if the situation gets serious you may sound out of context by saying it.
It's common to say wrong things in these cases.
The phrase con yo AFAIK is incorrect, there's no use really in the Spanish language. However, I heard some expressions like this example:
- A yo me gustaría viajar por el mundo.
- I would like to travel around the world.
Here, a yo is an informal and regional way of referring to the first person I / me (I heard these kind of expressions from people from Boyacá Department).
NOTE: This must be considered an optional answer without having evidence of usage or history background.