Why does Shakespeare use imaginary animals in Act 2, scene 3, and Act 3, scene 1, in Much Ado About Nothing?

I am rather confused by your question. I have just re-read
both of these scenes and can't find any reference anywhere to imaginary animals. Do you
perhaps mean real animals and birds? If you look at Act III scene 1 for example, there
are a number of references to birds that are humorously used to describe the gulling of
Beatrice. Let us look at those examples.

Firstly, as Hero
and Ursula await Beatrice, Hero describes Beatrice approaching like a
"lapwing":

For
look where Beatrice like a lapwing runs

Close by the ground
to hear our
conference.

Lapwings are
known for running along the ground to draw preadators away from their nests, and so we
can perhaps appreciate the awkwardness and funny way in which Beatrice is running across
the ground to "eavesdrop" on the conversation of Hero and
Ursula.

Secondly, Hero describes Beatrice (to her hearing)
as a "haggard of the rock," which refers to a wild hawk, emphasising Beatrice's "coy and
wild" spirits. Lastly, a reference is made to a way of catching small birds when Ursula
says to Hero, "She is lim'd, I warrant, you, we have caught her, madam." Bird-lime was a
sticky substance smeared on branches where the birds were likely to land to catch them.
The image refers to the way that Beatrice, like a bird, has been "caught" in the trap of
Hero and Ursula.

So, although there are no imaginary
animals, you might find it useful to analyse the use of animal imagery in these two
scenes.