the pony package was offered from 2001 or 2002. i dont remember which. it also adds chrome plated rims. the 99-00 has the moulding stripe along the side of the vehicle, the 01 doesnt, the 02-04 does. strange. the 99-00 has the different spoiler. the 99-01 can have the 15" rims. the 02+ has the rims where you can see the lugnuts. or the one with the pony centercap. um....some idiots (i've actually seen this once) write the year of their car across the top of the windshield. (it said 99 Mustang.....duh....) the 99's have a slightly thinner circle around the mustang, if it's the spec. ed. then you can tell by the color....the darker yellow "chrome yellow" was only produced on the newer body styles 99-halfway thru 00. the zinc yellow (mine) was produced halfway thru 00-halfway thru 03, then the new more saturated yellow is halfway thru 03-05 at least. there are a few other color modifications.
the longer you own a mustang, the more you pay attention to the details of the model years, the easier it will be to determine what model year that car is. thats pretty much what i did....looked at each new model year seeing the differences.
oh and engine wise, yours has the 3.9L engine vs our 3.8's, the 99-00 has a metal MAF, and interior wise, personally i think the 99-00 has a nicer center console. i've replaced mine with a 2000's.