Mylo died at 11 months old, the suit said. After his death, the suit said, Horton found antifreeze in the basement of the apartment.
"Despite the significant layer (of) dust, a fresh hand print was on the bottle, to indicate recent use," the suit said, adding that Mairs was in the basement and LaMaita was in Horton's apartment two days before Mylo died.
In a YouTube video description posted by Horton, she claims LaMaita entered the apartment with poised cat food and left it in the food dish.
Horton, who has since moved out of the apartment, is suing for thousands of dollars in damages and attorney’s fees.
LaMaita would not respond to requests for comment from The Oregonian.
The lawsuit, obtained by the publication, can be read here.