In an episode where the Utroms are escaping while the turtles fight Shredder under the TCRI building, Krang from the old Ninja Turtles series makes a very short appearance as an Utrom.
In the episode "Secret Origins" part 3, one of the Utrom creatures is named Krang. Krang is an "Utrom" type creature from the original series.
In "Secret Origins (3)" April is dressed as her original 1980s series version and portrays her as a news reporter for Channel 6 News.
Through out the series, there were a few references to the 1980s animated TV series. April dressed in yellow clothes imitating a TV reporter for Channel 6. Another is of rhino Gen dressed as Rocksteady and a warthog resembling Bebop. There are few episodes where the turtles are watching Channel 6 News (April works for in the original). There is an Utrom named Krang, and Hun even shouts the word "Krang" in "Exodus part 1."
This is Leatherhead's first appearance in the series. He was the creature that was walking through the smoke. But he doesn't reveal himself to the turtles till a later episode.