From what I get from show:
 
Baby = Baby Pony  
Child/Kid = Foal  
Girl = Filly  
Woman = Mare

How fast or slow they mature and if it seems like regular equine age rates or human age rates or a mixture of human and horse for some reason is a question I’d like answered eventually… :T

You can have a foal (baby/very young pony) who is a filly (female) or a colt (male), much like a child can be a girl or a boy. When raising horses, a young horse is called a filly or colt from birth until 3-4 years of age (though I suspect the ponies in My Little Pony age/mature more like humans do).
 
“Foal” is only useful if you’re describing a very, very young or baby filly or colt.

Judging by the Ponyville Schoolhouse’s school paper being called the Foal Free Press, “foal” is a blanket term for fillies and colts (at least in this series).

I wonder how would a fully grown stallion pony looks like if he still have his foal eyes

Ponies are not baby horses. http://wiki.answers.com/Q/Is_a_pony_a_young_horse

@bp: Dose it matter? no it’s not so run along now troll

I always thought the “baby pony” tag was for ponies who aren’t foals in the show who are drawn as such. Perhaps “babification” would be better?

I’m sorry, I can’t hear you over the HNNNGGGGHHHHHs and d’awwwwwws.

Since you found it necessary to make this an image, the problem in the second paragraph goes the other way. Someone may want to search for unicorns just like someone may want to search for fillies or foals.
 
I’ll give you a firsthand example of a comparable situation. One time I wanted to find picture of Pinkie as a pegasus, if all such pictures were tagged “pinkie pie” and “pegasus” it would be a simple matter. Otherwise it would mean trawling through several images to find the small few I wanted.
 
More tags is a good thing, there are no downsides to more tags. There are downsides to less tags on the other hand.

also, filly is not a blanket term  
filly is a female, colt is male