"This is a rabbit."
A lot of languages have two words: one like coney and one like rabbit. No one knows why. Some languages have just one of these today but you may find both if you look at old words.
Yes. Sadly there are very few speakers and so very little chance of a Duolingo course anytime soon. Slightly more chance of a Breton course, but not if the French government has anything to do with it.
For the moment just cùm ort leis a' Chuimris keep going with the Welsh (I wish I could say that in Welsh).
These colours have been discussed at https://forum.duolingo.com/comment/35871246?comment_id=35887702 and https://forum.duolingo.com/comment/36391243?comment_id=37460343
Basically, glas meant the same in all Insular Celtic languages, until they tried to match it up with some English word in the different languages.
If you mean that coney is a word that was more common in the past, you are right, but as my links in a previous post show, you still get coney about 1% of the time.
However, the term Old English is reserved for the Germanic language spoken in parts of England and Scotland until it merged with Norman French to form Middle English shortly after the invasion in 1066. As far a I can ascertain, no one knows a specific term for rabbit in Old English. (I guessed what word they used but I edited it out when I realized they did not have rabbits in pre-conquest England.)