There is often more than one way to do things. It would depend on the method described (if the designers wanted those effects as Rego Animal, they would be). Muto also covers change from one state to another natural state, such as gender change, a face change, etc. These are all normal "end" states, but the process of altering the body is unnatural because of the speed of the change or the change in the "original form" of the body.
I agree Rego could do this easily as well, but Rego could also not allow a change in humor too far above what a body could normally produce. So Rego has a logical "soft cap". Creo can also artificially increase the humors present directly; and probably far further than any other art. Creo is probably the easiest in terms of magnitude too. It is not as simple as saying "I want +2 strength" the spell has a dependency on understanding how the Magi is doing this.
Further I'd argue that I could conjure an natural additional organ in the target's body which creates that humor, just like I could conjure a set of animal wings, or an additional arm.