We do not have a goat lineart, so whats why it doesn't show up in the archives under species. There have been several times when an existing lineart (like deer or pony) have been edited to look like a goat, so thats probably what you saw. Are the ones you are looking for look anything like these?
These are from Lunar New Year 2015 (year of the goat so we got a bunch of goat/ram/sheep themed pets and items)
These are on the deer and pony linearts
Archive link:
archive/2015/Chinese%20New%20Year/We also sometimes get some goat/sheep themed pets on Easter, this one is from Easter 2014 and its a modification of the butterfly wolf lineart;
Archive link;
archive/2014/Easter/Butterfly%20wolfI believe there are more but thats just from a quick look in the forums. If you remember any specifics of what they looked like that will help a lot in finding them ^^