Quarter guard
Two different positions fight over this term.
Gustavo Machado put out a "quarter guard" instructional that covered a half guard position where you wrap one of their arms with your shin against their biceps, like this:
The other position claiming this name is when someone has gotten their knee out of your half guard and you're hanging on to their ankle. It's half of a half guard, so "quarter". Eddie Bravo uses this term in all of his books so it's more popular.
Found a little video teaching it.
Z-guard
From half guard, bring your knee across their hip, stomach or chest like you're setting up a scissors sweep. Leo Kirby coined the term as a joke but then it stuck. Here he is showing it:
Reverse De la Riva
From the same position as normal De la Riva, bring the opposite foot than normal and hook under their knee. Here is Saulo teaching it in his first set:
There are even more unnamed positions and grips and variations of all of these, and it gets even more confusing when you start using them in combination and transitioning between them freely.