The spell legendary proportions says that it can only target creatures. What, exactly, counts as a creature? Are PCs creatures? Are all living things creatures? Or are creatures only nonsentient things like horses?
Anything that has a Wisdom and Charisma score is a creature. Anything that lacks them is an object.
That means not only any living being, but also a bunch of non-living ones; Undead are creatures, as are Constructs and Elementals.
So yes, you can use such spells on players, other humanoids, animals, etc. The spell specifically calls out that it doesn't work on Undead though; which shows that Undead are normally also considered creatures, just this spell won't work on them.
(Note that a lot of spells target creatures, many of which you'd expect to be able to use on just about anything. Magic Missile for example also targets creatures, you would not expect it to only work on animals, right?)
Quoting from the SRD on abilities:
Every creature has a Wisdom score.
Every creature has a Charisma score.
In the notes for both:
Regular plants, such as one finds growing in gardens and fields, lack Intelligence, Wisdom, and Charisma scores; even though plants are alive, they are objects, not creatures.
It's spelled out even more literally in the D&D 3.5e SRD on Wisdom (& the same for Charisma):
Any creature that can perceive its environment in any fashion has at least 1 point of Wisdom. Anything with no Wisdom score is an object, not a creature. Anything without a Wisdom score also has no Charisma score.