A big fish in a small pond
is an expression about an important person in a small place.  Do fish only grow to the size their environment is able to sustain?

Battered fish are fried and appear on a menu or in a fish and chip shop.  (Batter is made from flour, egg and milk.)

A slippery as an eel describes an evasive person (who cannot be pinned down either physically or figuratively).

A silly goose is sometimes a gentle way of telling a child they have done something daft.

When the fox preaches beware of your geese is an expression I came across as an adult.  Is it about false prophets?

The rat race describes human society involving commuting, work and the pressure of trying to climb a career ladder.

Are you a man or a mouse?
This rhetorical question is an idiom.  It can be addressed to any timid individual to encourage them to take a bolder course of action.

I am a bear of little brain and…… bother me.

Winnie-the-Pooh said,

I am a Bear of Very Little Brain and long words Bother Me. 

(The House at Pooh corner by AA Milne).  It is frequently adapted to fit other situations in a jokey way.

No use shutting the stable door after the horse has bolted.
This is a proverb with a similar meaning to prevention is better than cure.

You can’t change your horses in midstream.
When a stream or river is so deep that a horse needs to swim a rider could not possibly change horses without risking death by drowning.  The expression is used about changing one’s mind after setting out in a particular way.

A nod is as good as a wink to a blind horse – it wouldn’t see either.  This is said when someone has been given a hint, to indicate that they have understood the message.

On your high horse is defined in the Concise Oxford Dictionary as “behaving with pretentiousness or arrogance”.

A stubborn as a mule is a simile.  A mule is a cross between a horse and a donkey – it has a reputation for being difficult to manage.

Shanks’ pony means on foot.  The Dictionary of Modern Phrase by Graeme Donald gives the derivation from shank being a word for leg.

A herd of goats is the collective noun.

Herded together may be applied to people in a confined space.

Flocked to see is an expression about crowds of people.

A flock of sheep is the collective noun.  Flocks and herds are mentioned many times in the Bible.  One example is Psalm 8:7.

It also talks about the One who was led like a sheep to the slaughter.  Acts 8:26-40