Let the cat out of the bag.
Give away a secret.  Obviously not a transparent plastic bag!

A cat may look at a king.
This expression is so familiar it is hard to find an off the cuff comment for it.  Some people do have a problem with being watched 

While the cat’s away the mice will play.
Or while the boss is out the workers might shirk.  St Paul had something to say about this in his advice to slaves. Ephesians Ch6 vv5-8

As quiet as a mouse.

Eek!

You can’t teach an old dog a new trick.
But you are never too old to learn.

Give a dog a bad name…
People do tend to believe what they hear about others and especially what you tell them yourself.  After all they would have
heard it from the horse’s mouth. 

See a man about a dog.
A useful expression when you prefer not to be specific about your intentions.

It’s a dog’s life.
A working dog or a mistreated dog or a pampered pup.  Now there’s a phrase to interpret! 

You’ve been sold a pup.
Presumably it was described as a well-trained dog.

Don’t make a pig of yourself.
Don’t be greedy. 

If pigs could fly…
A more gentle way of pouring cold water on someone’s ideas.

Buy a pig in a poke.
A poke here is a bag and comes from the same root as pocket (a little poke).  If you haven’t seen what you are buying you may be disappointed.

You can’t make a silk purse out of a sow’s ear.
Always use the right materials and don’t expect people to be what they are not.

Like a red rag to a bull.
Inflammatory.

Till the cows come home.
Presumably if the farmer didn’t go and get the cows at milking time, they would not bother on their own.

For donkeys’ years.
I don’t know how long donkeys live do you?  This is always used to indicate a long time.

Donkeys go best laden.

When there was rather too much to carry, this was the comment to make resignedly.

As mad as a March hare.
Over-enthusiastic, perhaps.

Sky blue pink- the colour of a rabbit’s egg.

No such colour, no such thing.  I always felt confused by this and the adults using it seemed to think it was a big joke.

To be the bee’s knees and the cat’s whiskers.

Or the cat’s pajamas like this blog heading, which I haven’t found out how to edit!  To be the best. 

Behave!  (Abbreviated to bzz in company to convey the buzzing of bees in a hive!)
This was a private communication in our family.  The local accent tended to pronounce “Behave” as “Behive”, which was nearly beehive.

I’d like to be a fly on the wall.
To be an unnoticed eavesdropper.

She’d skin a flea for its hide.
This is taking thrift and re-use of materials to extremes.

Bright eyed and bushy tailed.
A squirrel, perhaps.  Ready for anything