Textbooks on cosmology describe the phenomenon of cosmic inflation in terms of the existence of a scalar field (or many scalar fields, as the answer and comments pointed out), called inflaton. Such as choice without motivation seems ad hoc. Is there a reason why inflaton should be a scalar field (instead of a fermionic or vector field) apart from making the theory simple?
There are hundreds of inflationary models, and some of them indeed use vector fields (I'm sure you can come up with spinor models as well). But then you can ask yourself - why describe inflation with "100" scalar fields, when you can just as well describe it with only one? Of course you should keep in mind those other possibilities, but lean towards the simplest ones.
Good textbooks make it clear that inflation is a paradigm, not a theory, and focus on general aspects of inflation that are common to all possible models.