Who doesn't remember them? The egg shaped keychains that hide in their belly a cute animated scene?
Originally they were brought out by Takara in 1993 but later on they were remodeled into a newer shape by Z Wind Ups although personally I do prefer the ones from Takara. Sorry Z Wind Ups.
Takara brought out several series of these egg shaped keychains. You had categories as:
- sea
- jungle
- farm
- family
- tropical
- chicks
- ...
And that is only counting the ones in which animals play the staring role. Takara also brought out pocket critters around themes as:
- sports
- tv shows (Babar or Astro Boy for instance)
- people (a hula dancer for instance)
- ...
And even cooperated with other brands for unique pocket critters like the ones made in association with Sanrio for Hello Kitty.
Some examples:

Takara examples for the jungle assortment

Takara examples from the assortment cats, dogs, pigs and farm

A few examples from Takara's chicks and bunnies assortment

Takara's sports assortment

Barbar from Takara

Takara's pocket critter made for Coca Cola

Takara and Sanrio joining forces for pocket critters Hello Kitty

Astro Boy from Takara

Z Wind Ups Dinos

Z Wind Ups Dolphins

Z Wind Ups Cows
Please also note that Takara themselves too have brought out some of their pocket critters anew in a slightly different version. For instance the "Mooo" pocket critter was later replaced by the "Farm" pocket critter:

The original pocket critter for the cow from Takara

The later brought out pocket critter by Takara for the cow
Site of Takara with some listed pocket critters:
http://www.javys.com/takara/Pictures on my Pinterest account of the ones I have found so far:
https://www.pinterest.com/faoabt/pocket-critters/ (note that I am only interested in the "animal" pocket critters and a few of the "special" ones. The "people" pocket critters are passed by by me)