Spanish Yo/Llo

I'm teaching math review courses for different grade levels right now for the summer at Santiago Canyon College. Today was the first day of class, so it wasn't planned to be that rigorous. Anyway, the topic of language was brought up, where one student said they only spoke English and that Spanish was easier than English. I decided to insert my foot in my mouth by talking about something I didn't know a lot about: Spanish.

As a counterexample to what the student said, I asked one of the Spanish-speaking students how one says "I like it" in Spanish. Naturally, they replied, "Me gusta". I then explained how the phrase's construction is different than the way it is in English, and counter-intuitive for someone learning Spanish who knows English, and surely they would have a problem flubbing that expression. Yet I also stressed that for someone learning English who knows Spanish, it would be counter-intuitive to say "I like it," and they also would have the same problem as the Spanish-learner. The whole point is that each language has its complexities and no language is "simpler" than another, although each language may be simpler in a certain linguistic category, e.g., phonetic inventory.

I then explained that some things are just as simple in English as Spanish. I asked a Spanish-speaker how to say "I am eating food". They replied, and I transcribed it on the chalkboard as "Yo como comida". However, when I wrote the "Yo", both Spanish-speakers in the class protested that that's not how you write it, which was a real surprise to me, since I thought that part of my Spanish knowledge was sound. When I asked them how they write it, they both spelled out "Llo." I had never heard of that form of "I" in Spanish, so I asked where their families came from: one's family was from Mexico on both sides, the other's had one side from Mexico and the other from Cuba. So that just really stumped me, because I thought if I was familiar with any sort of Spanish, it would have been Mexican Spanish.

So when I got home today, I decided to google the "Yo/Llo" thing.[1] Unfortunately, I only found three sites (here, here, and here), that even mentioned "Llo". So where in the heck is "Llo" coming from?

[1] The search terms I used:

llo yo
llo yo spanish
llo yo espanol first person singular pronoun
"Yo llo"

Update:
I tried searching with:

yo llo orthography
yo llo spelling

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