Both forms of English prefer fledgling to fledgeling, but ridgeling to ridgling.
From the en.wikipedia.org
An example of a ridgling that was gelded was the Kentucky Derby winner Funny Cide.
From the en.wikipedia.org
A total of 371 colts, 25 geldings, four fillies and one ridgling were nominated this time around.
From the fresnobee.com
A ridgling or rig is a male animal with an undescended testicle.
From the en.wikipedia.org
The 7-year-old Blissfull Hall ridgling set all the fractions after assuming command from Real Magician early.
From the delawareonline.com
More examples
Ridgeling: a colt with undescended testicles
A male equine with one testicle.
A horse with one or both undescended testes.
Term describing either a cryptorchid or monorchid. Also spelled "ridgeling."
(or rig). A male horse which has has an undescended testicle or has been incompletely castrated is called a ridgling. It will appear to be a gelding, but will have some of the behaviours of a stallion.