The latest episode of Batwoman confirmed that Killer Croc exists in the Arrowverse. Batman may have mysteriously disappeared in the Arrowverse, but viewers are getting a strong sense of what Gotham was like under the shadow of the Batman. This version of Bruce Wayne has been active in Gotham City for at least 15 years - probably substantially more.
As a result, Batwoman is deliberately name-dropping a lot of Batman characters. Kate Kane's origin story has been rewritten to incorporate the Joker, Robin got a reference in Batwoman episode 2, and Tommy Elliot - aka Hush - was the main villain of Batwoman episode 3. This version of Gotham is clearly well-populated by members of the Dark Knight's rogues' gallery, which means Kate Kane's becoming Batwoman could draw any of them out.
A throwaway line of dialogue in Batwoman episode 4 confirmed that Killer Croc exists in the Arrowverse as well. The episode saw the sneak-thief Magpie continue to steal from the rich and privileged in Gotham, but Batwoman was otherwise engaged at the time; Kate was in bed with her new girlfriend, Reagan. The couple switched on the radio to hear Vesper Fairchild calling out Batwoman's absence, and Reagan cracked wise about having heard Batwoman was "doing" Batman. An awkward Kate shot back with as much witty repartee as she could manage, asking Reagan who she'd heard that from - Killer Croc?
It's only a throwaway reference, and it isn't necessarily setting anything up, but it clearly establishes that Killer Croc exists in the Arrowverse. In the comics, Waylon Jones was born with a rare medical condition that imbued him with reptilian traits. He was originally depicted as a normal human with scales instead of skin, but over the years has effectively mutated into a monstrous, bestial form known for eating people alive. The comics have explained this away by suggesting that Killer Croc's disease has progressed, increasingly robbing him of his humanity.
Killer Croc made his big-screen debut in 2016's Suicide Squad, where he was played by Adewale Akinnuoye-Agbaje. Although Killer Croc was quite a popular character, he's not expected to return in James Gunn's sequel/relaunch, The Suicide Squad. Given that's the case, it is possible the supervillain will now be available to the Arrowverse. The Flash has successfully pulled off both Gorilla Grodd and King Shark, proving they have the technology to make this kind of monstrous being work. Only time will tell whether or not Kate Kane winds up taking on the Arrowverse's Killer Croc in Batwoman; if she does, it's quite possible she'll realize that he's no laughing matter.