All so much of the inexpensive restaurant food in Guatemala is the same. That is, eggs (how you like them), plantain, beans and tortillas (with an optional meat choice),cheese or crema,  tortillas and coffee for breakfast. The exotic-sounding alternative is "pan frances" which is nothing like French bread -- just white bread rolls. Most lunches are fried chicken with french fries. Dinner could be like breakfast or like lunch.

Nicaragua and Costa Rica at least offers gallo pinto as an alternative (details of gallo pinto in a post soon) but not here. Non-local food options tend to be only available in more expensive places. But, the good news, is that pupusas are available in Guatemala. While "foreign food" they are from El Savador and you find salvadoraños preparing them so they tend to be inexpensive.

What are Pupusas?

My quick description is they are like a pancake made from corn flour (or, sometimes rice flower) stuffed with pretty much anything. Wikipedia actually does a good just of explaining what they are. The most common ones you find here are stuffed with cheese, cheese and loroco, chicarrón or beans). They are served with a tomato sauce and a spicy cold slaw-like concoction.

I say "most common" but there are alternatives. For example, Chero's Bar (chero is slang for friend in El Salvador) in Panajachel has a great assortment. The wife of Ricardo, the owner, is from El Salvador. They offer about 20 options, about half being Vegetarian.

Where can you find Pupusas?

Don't go looking for a fancy restaurant. Small restaurants and even street carts have time. Here in Zona 1 of Guatemala City, there is a place on 7th Avenue at about 13th Calle that sells them and only them. Just north of Plaza Barrios (near the terminal for the 6 and 18 Transmetro lines) there is a cart that has them at a really cheap price (as I remember, two for Q5). There is also a small restaurant with hot dogs that also has them on 11 Calle just west of 9 Avenue.

Give them a try. It's a nice alternative from "comida typica".