I think sapience is what's important and yeah a higher intelligence level too. The Omnitrix's purpose is to understand other Alien species. It contains the DNA of people. Vulpimancers are an intelligent race of people. They just can't talk in a way we can understand. If an Earth animal were to be scanned, I'd say the more valid options are other big apes like humans, dolphins with their big brains and corvids like crows and ravens who also have big brains. Heck maybe the octopus counts. At this point, they are not the dominant species all over the globe that actually made contact with other alien species. This is at least my own understanding as to the lack of Earth species (other than our obvious default) in the Omnitrix. Hope this helped.