Ok, I've seen a lot of not-so-in-depth suggestions for pets, so I'll try to make mine have as much detail as possible.

There would be three ways of getting these pets.
1) Through a special merchant. This merchant will only sell basic pets like shuffle-bots and snipes of varying colours. Probably for crowns.
2) Picking up the dead carcass of defeated monsters. Sounds a bit gruesome, but the icon will be like the materials icon. There will be a VERY small chance of the carcass appearing as a drop. Once you have the carcass, take it to the same merchant and spend (probably 300) energy to revive it as a pet.
3) Through mail for special events. Like the Gourdlings for the dark festival etc.

ALL pets will be purely for looks.
The pets can always be out (even in the clockworks) and will just stick around near your body if you lose all your health. (Except PvP)
As for the entities, you can walk through them.
You can only have one out at a time.
Not sure whether to make them trade-able over the AH. Probably yes.
The pets will go in a separate slot for your spiral knight.

I think I have covered everything. This idea is open to CONSTRUCTIVE CRITICISM and ideas.

Fire away.