Totto is baby language for 'horse'. Probably onomatopoetic, after the sound of a trotting horse.
Incidentally (I think) there is also the word toto, short for totalisator, a system for gambling at horseraces.
Another, more common pet word for 'horse' is pålle. No idea why! Pålle comes from Pål which is the Swedish version of Paul(us).
Mer om hästar/More about horses: http://www.svd.se/kulturnoje/serier/berglins_5017.svd