The community of An American Tail fans is referred to as a fandom. While not a large community, the fandom of An American Tail has manifested itself in many ways on the Internet over the years. It mainly consists of adults who grew up with the movies, though many on the younger side who watched the movies later are also a part.
See also: Notable Fan Works
Fans of An American Tail and its sequels in the 1980's and 90's were young for the most part and not really organized in any way. The first movie was a record-breaking success at the box office in 1986, creating a lot of fans, and Fievel Goes West, while less of a financial success, became so well-known that it eclipsed the first film for a few years and many kids forgot that it was a sequel to anything. The movies got a mixed reception by adults.
Notable Fan Sites
Many fan sites emerged in the late 90's and early 2000's, most of which were on Geocities and aren't available today. A few of note:
This fan site was one of the most notable An American Tail fan sites in the early 2000's. Its maker, screen name "GokuFievel", was a fan of both anime and Don Bluth movies. GokuFievel reviewed all four of the movies, and even got a very rare interview with Phillip Glasser. The site was active until about 2006.
This website, started by a user with the screen name Surenity, was active between 2009 and 2013, and featured comprehensive reviews of the movies, and Fievel's American Tails, profiles on cast and characters, a gallery, as well as a forum community. It was created using Webs.com (formerly freewebs), which placed strict limits on how large the site could be and how much bandwidth it could use. Trouble with bandwidth caused the site to become frozen and taken offline often, so it was split in two to remedy the problem, however troubles persisted. This very Wikia was created to be a spiritual successor to the fan site, able to carry much more information without crashing, and many articles from the fansite were transferred here.
From 2006 to around 2011 there was a thriving roleplay community on the social networking site Myspace where people would start profiles for fictional characters, and An American Tail characters were popular among the cartoon roleplay community, which followed a storyline similar to Who Framed Roger Rabbit. This community eventually died out after Myspace ushered in unpopular changes and people left for Facebook, however.
This proboard forum was the main gathering for fans of the pairing of Tanya Mousekewitz and Cat R. Waul, a popular subset of the fandom, from May 2008 to it's last posting to date in 2013, started by a user named Tanya M. Waul. Of note was this forum was one of the first places on the internet to post episodes of Fievel's American Tails, making them more available to the fandom at large since they are not available on DVD.
Never Say Never Again -DeviantArt Group
This DeviantArt group, a spin-off of the fan site listed above, was started on May 17, 2011 by users BTM05, Maxl654 and Surenity, with the main goal of collecting every piece of An American Tail fan art and fan fiction from the site into one place. On the 25th anniversary of An American Tail it held an art contest that received many submissions, and another was announced for the 30th anniversary of 2016. The winner of the contest, WhiteLionWarior, was allowed to have their anniversary artwork as the main image of this wiki. Unlike the fan site it came from it is still active and accepting submissions.
Another DeviantArt group started by user Harmony-Borealis, this is the main center of the popular crossover pairing of Fievel Mousekewitz and Olivia Flaversham from Disney's The Great Mouse Detective. It collects artwork of either Fievel or Olivia, or both.