United Airlines further tightens rules on emotional support animals The rules will not affect those who want to bring service dogs on board.

United Airlines is the latest carrier to tighten its rules on emotional support animals, limiting them to flights under eight hours, the airline announced on Thursday.

"We have seen increases in on board incidents on longer flights involving these animals, many of which are unaccustomed to spending an extended amount of time in the cabin of an aircraft," United said in a statement to reporters.

The airline also does not allow dogs or cats under 4 months old on board and limits emotional support animals to dogs and cats. Miniature horses are still allowed as service animals.

In December, Delta Airlines updated its service and support animal policy to ban service and support animals under 4 months of age, regardless of flight length, and limit emotional support animals to flights under eight hours.

ABC News previously reported that Airlines for America, an airline trade association, estimated that from 2016 to 2017 the number of emotional support animals aboard U.S. commercial flights grew from 481,000 to 751,000.

In 2018, United revealed it saw a 75 percent increase in a single year, prompting a new requirement that emotional support animals have a health and vaccination form from a veterinarian, in addition to assurance that the animal could behave.