Toto is a K9 dog that traveled with Dorothy Gale to the Land of Oz.
In the world of Emerald City
It's assumed Toto is a male police dog as his gender has not yet been mentioned. The name 'Toto' was given to the dog by Ojo, because it means 'dog' in Ozian language.
Toto, being a police dog, is quite intelligent. He joins Dorothy on her journey and it looks like he also accepted Lucas in his 'herd'.
Toto is protective and vigilant, but it seems he's not sensitive to magic, as East manages to sneak upon Dorothy on his watch.
In the works of L. Frank Baum
In the books, Toto is a dog of a small size, with black fur and eyes, probably of terrier breed.
He is transported along with Dorothy to Oz and accompanies her on her journey.
Coincidentally, being transported to Oz gave Toto the ability to talk and understand human speech, as with the other animals in the Kingdom. He just choses not to reveal this ability.
- "Toto" is not the dog's real name, but a Munchkin's word for a dog, according to the first version of the script.