(written from a Production point of view)
April Tatro is a former contortionist and actress who appeared in the role of Isis in her Human form in the Star Trek: The Original Series second season episode "Assignment: Earth". She received no credit for this appearance and filmed her scene on Friday 5 January 1968 at Paramount Stage 5. 
In fan circles it was long speculated that the briefly seen Human form of the cat Isis was portrayed by actress Victoria Vetri, and this had been repeated so often that many articles and websites treated it as fact. However, Vetri herself confirmed that she was not in the episode: "I was never in an episode of STAR TREK. I know that people think I was […] I am not sure who she was. Look close enough and you can see that she has blue eyes and I, of course, have brown." (Little Shoppe of Horrors #41, p. 71)
On 12 March 2019 (citing a production call sheet for extras dated 5 January 1968), Larry Nemecek revealed on his podcast The Trek Files that the Human form of Isis was portrayed by actress, dancer, and contortionist April Tatro. 
Beside her appearance on Star Trek, Tatro was featured in episodes of Wonder Woman (1976, with Wayne Grace), Totally Hidden Video (1990), Matlock, Johnny Bago (1993, with Larry Hankin and Joe Gieb), Family Matters (1997, with Jeff Coopwood and Cynthia Graham), Ellen (1997), Malcolm in the Middle (2001), and Days of Our Lives. 
In addition, she appeared in shows such as The Tonight Show Starring Johnny Carson (1968), Fernwood Tonight, and Laugh-In (1971 and 1978, with Kim Braden), performed stunts in the Western Blazing Saddles (1974, with Bill Catching, Dick Crockett, Andy, Gary and Tony Epper, Robert Herron, Buddy Joe Hooker, Julie Johnson, Bill McIntosh, Hal Needham, and Paul Stader), and appeared in the 1993 music video for the Brother Cane song "Hard Act to Follow" and in the 1997 music video for the Aerosmith song Pink.  
Among her film work are the science fiction comedy What's Up, Hideous Sun Demon (1983, with Googy Gress, Mark Holton, and Tony Plana), the comedy Big Top Pee-wee (1988, with Albert Henderson, Jack Murdock, Kenneth Tobey, Jay Robinson, and Kevin Peter Hall), the television comedy The I'm Exhausted Concert (1988), the Western Silent Tongue (1993), and the television movie Bandit: Bandit's Silver Angel (1994, with Christine Jansen).