Tony Rivera was Grim Natwick's assistant on Snow White. "Tony Rivera, a very good artist, cleaned up some of my drawings later, towards the end to finish up the picture - to get it on the road." According to Leonard Maltin's "Of Mice and Magic" he was "hired off the picket line" at Disney in 1941 by Frank Tashlin when Tashlin was assembling his Screen Gems staff. He also worked at UPA, DePatie-Freleng and H-B where he was, among other things, the catalyst for something known as "The Drop".
Franzen, with an evident and obvious pride, especially recalls something the artists called “the drop’. The animators worked on the second floor of a building they affectionately called “the warehouse”. Below them worked the cel painters. In stark contrast to the wacky world above them, absolutely no talking was allowed among the cel painters. They worked in cloisterous icy silence. To the animators this presented a challenge! For a while Franzen was not aware of the triggering mechanism, it seemed that at random intervals during any given week, suddenly a “drop” would occur. It started simply enough. Pencils. A coffee cup. Then more coffee mugs. Then larger objects… all simultaneously raining down on the floor in a violent cacophonous crescendo that always culminated in the dropping of a large metal sign and finally several brave souls lifting up and dropping an actual animation desk to the floor. This of course led to an explosion of boisterous laughter and applause… followed by silence and footsteps. The footsteps were those of the head cel painter as she headed upstairs to the second floor. She was a hard woman with no sense of humor, and every time she would appear on the second floor after a “drop”, she would be greeted with a combination of blank stares, muted smiles, and denial as she demanded, “What is going on up here?! It was months before Jim Franzen, the new guy on the second floor, found out that the trigger for these “drops” was simply the random sneezes of fellow animator Tony Rivera! How this tradition started remains a mystery even to this day…
I haven't found any information about what he was doing at MGM in 1944.
In 1953 Rivera was production designer on GE's "A Is For Atom", working with animators Emery Hawkins and Arnold Gillespie. Gillespie, another former Disney animator, worked at Fleischer's on Gulliver's Travels.From an interview with animator/comic book editor Vince Fago:
We were required to do thirty feet of animation a week. Arnold Gillespie, who had worked at Disney, said he could do thirty feet a week but it wouldn't be his best work. He said he could do twelve feet and it'd be great. So they let him, and we worked together. We were the only guys working on that quota. Gillespie was also the first man to do pencil tests. We shot the scenes in pencil to see how it'd look.
In 1944 Gillespie was animating Barney Bear at MGM. (Maybe Rivera was too.) IMDB lists his credits under A. Arnold Gillespie, a long-time art director and special effects artist at MGM. In 1944 A. Arnold won the Oscar for Best Special Effects for "Thirty Seconds Over Tokyo." It seems certain that they're not the same guy.