Crisis on Infinite Earths was one of the landmarks in comics history. It was the first time that the publisher did a massive reboot on the history of their heroes. It also saw a commitment to change with the deaths of both minor and major heroes like the Flash and Supergirl. On top of everything else important about this event, Crisis on Infinite Earths contained almost every character published by DC Comics to that point.
While so many heroes played a part, some showed up for a minute and were gone. While Hal Jordan wasn't seen at all during the twelve-issue series, there's a number that did show up that people have forgotten about.
10 Starman (Prince Gavyn)
In 1985, the legacy of Starman was not very strong. The Starman of the Justice Society had no Silver Age counterpart. In 1980, Paul Levitz and Steve Ditko introduced a new, alien Starman in the pages of Adventure Comics. He had a journey from spoiled prince to the masked protector and then ruler of his homeworld.
Crisis on Infinite Earths revealed that Starman died protecting his people from the anti-matter cloud. It was a tragic and pointless death that James Robinson undid in 1999, working in his death to the origin of a new Starman. This all culminated in the return of Prince Gavyn.
9 Tawky Tawny
If there's a character that personifies the fun comic book nature of Shazam stories (then known as Captain Marvel), it's Tawky Tiger. A talking tiger is one of the things that used to happen only in comics. Efforts have been made to update him, making him look more like an actual tiger, but the original, humanoid tiger in a suit always had a charm that can only be found in those earlier Shazam stories.
In Crisis, Tawny shows up after Earth-S is merged with Earth 1.2.4. and X. It's a beautifully ironic scene that not only highlights the innocent absurdity of Earth-S but the irony of Changeling being shocked by a talking tiger. He later shows up as a captive of the villains that have conquered Earth-S.
8 Tawny Young
Tawny Young was pretty much a forgotten character until DC Universe's Harley Quinn series brought her back as a talk show personality. Like with most characters in animated DC projects, Tawny Young came from the comics and was a west coast reporter that showed up repeatedly in Green Lantern. She most recently showed up in Rob Vendetti's Green Lantern series.
Tawny showed up in Crisis reporting on the crisis itself from a journalistic perspective, alongside Lois Lane, Lana Lang, and others. Her inclusion helps to give a perspective of the crisis from the average civilian in the DC Universe. The fact that Harley Quinn has brought her back to the forefront a little helps breaks the perception that all journalism in the DC Universe is relegated to the Daily Planet.
7 The Inferior Five
This was truly a cameo in the last issue. As Lois Lane is reporting on the death of Dove, from Hawk and Dove, we see several super-heroes behind her, helping civilians. In the second shot, we see Infinity, Inc. In the first shot, we see Merryman, Awkwardman, and Dumb Bunny from the Inferior Five.
Most fans may not remember the Inferior Five outside of the most recent short-lived series from Keith Giffen. Originally, they were the children of the heroic Freedom Brigade. They have popped up from time to time in DC Comics, mostly in more humorous stories.
6 Angel O'Day (From Angel And The Ape)
While her simian partner isn't seen with her, the eleventh issue of Crisis saw a convention of detectives come across the mysterious corpse of Wonder Woman villain the Angle Man. Sadly, nothing much came from it.
Angel and the Ape was a more humorous series featuring the detective and her partner, Sam Simeon, a hyper-intelligent gorilla. One post-Crisis retcon had Sam as a former citizen of Gorilla City. The last time that they had any adventures of their own was in a four-issue limited series from Vertigo in 2001.
5 Proty II
The former pet of Chameleon Boy from the Legion of Super-Heroes showed up twice in Crisis. The first time was during a crowd watching stampeding mammoths. The second was perched on Legion equipment near Chameleon Boy.
Proty II had ceased being Chameleon Boy's pet at this point. He had been working as an event "photographer." There was an underlying social movement for recognition of his race as sentient. Sadly, the sentient rights movement wasn't developed much more, and neither was Proty II.
4 Black Orchid
In 1985, Black Orchid was more or less a forgotten character. She had gotten a full-page entry in Who's Who, but had not appeared since 1976. She was a mystery, but one that was never unraveled before the Crisis.
Writer Neil Gaiman brought her out as a character. He redeveloped her as a human-plant hybrid, similar to Swamp Thing and Jason Woodrue. It left two surviving plant-beings to carry on the legacy of Black Orchid, one significantly younger than the other.
3 Vandal Savage
Vandal Savage first showed up in the past, observing the Monitor's machinery during the age of Camelot on Earth-2. Next, he is seen in the gathering of super-villains on Brainiac's ship. Most likely, this is a more current Vandal Savage.
Vandal Savage never became a major threat in the DC Universe, although his appearances were plentiful. It wasn't until well after Crisis that he became one of DC's more dangerous villains. It's his prominence in future stories that make his cameos so extraordinary. He doesn't play a major role at all in the story, but given how he's been portrayed since, he plays the long game.
2 Swamp Thing
Given the visibility of Swamp Thing in the DC Universe, it's hard to believe that he wasn't a bigger part of Crisis on Infinite Earths. At this time, the character was amid redevelopment by Alan Moore that set the stage for taking the character in a new direction.
Swamp Thing does show up in the Monitor's satellite with the rest of the heroes and villains from Earth-1 and Earth-2. He makes a statement to the soldiers of Easy Company and vanishes until he reappears during a recap of the crisis by Harbinger.
1 John Constantine
John Constantine has become an important character for DC Comics, first as one of the anchors for the Vertigo imprint. In 1985, he had just been introduced by Alan Moore in Swamp Thing. His appearance in Crisis just emphasizes that the crisis is affecting every corner of the DC Universe.
Coincidentally, his appearance in Crisis on Infinite Earths #4 shipped two weeks before his official first appearance in Swamp Thing #37. That's the reason that his manner of dress differs so much and lacks his trademarked trenchcoat. He doesn't show up again in the event, something he has in common with many cameos.