Nope. Not a fungus, although quite a pretty electron micrograph below, wouldn't you say? Everyone knows it already: the compound found in the pet food is called aminopterin. It is a rodenticide that is not approved for use in Canada or the US. Aminopterin has a similar mode of action as methotrexate, a commonly used chemotherapeutic drug. The typical rodenticides (at least those known to vets) inhibit vitamin K synthesis, important in blood clotting. This is why veterinary patients can hemorrhage after ingesting this type of rodenticide.
Rodenticides causing kidney failure (acute tubular necrosis, more specifically) was hitherto (who am I? Poe?) unknown to me.
Aminopterin, not a fungus.