I thought I'd make a central post for all my growing, breedable adoptables. So here it is.

Dragons:
http://www.furaffinity.net/view/4905329/ (4 eggs)

Dragon eggs cost $1. Additional growth stages cost $8.
There are 5 additional growth stages, but only 4 required for breeding.
Breeding is free, and produces 6 eggs.

Dragon examples:
http://www.furaffinity.net/view/4952409/
http://www.furaffinity.net/view/4952882/
http://www.furaffinity.net/view/4952992/
http://www.furaffinity.net/view/5324139

Ponies:
http://www.furaffinity.net/view/4997165/ (4 eggs)

Pony eggs cost $1. Additional growth stages cost $8.
There are 2 additional growth stages.
Breeding is free, and produces 6 eggs.

Pony examples:
http://www.furaffinity.net/view/5011643/
http://www.furaffinity.net/view/5369238

Mythical:
http://www.furaffinity.net/view/5006447/ (8 eggs)

Mythical eggs cost $1. Additional growth stages cost $5
There is only one additional growth stage, ie. they hatch into adults.
Breeding costs $1 per egg, unless cross-breeding with a dragon or pony, in which case 3 eggs will be produced for free.

Mythical examples:
http://www.furaffinity.net/view/5006458/
http://www.furaffinity.net/view/5026761/
http://www.furaffinity.net/view/5066280/
http://www.furaffinity.net/view/5083309/

Flexible fantasy:
http://www.furaffinity.net/view/5380626/ (3 eggs)

Fantasy eggs cost $1. Additional growth stages are name your own price.
There are as many growth stages as the buyer wants.
Breeding costs $1 per egg, unless cross-breeding with a dragon or pony, in which case 3 eggs will be produced for free.

Flexible fantasy examples:
http://www.furaffinity.net/view/5942578/
http://www.furaffinity.net/view/5646114/

Also, anyone who is interested in the idea but doesn't like any of the available eggs can request additional eggs of any type and I'll happily produce some more!