"The chicken rode in the bus."
Translation:Das Huhn fuhr im Bus.
If you told me "Ein Hähnchen fährt im Bus", I'd imagine it to be a fried chicken, while I'd imagine "Ein Huhn ..." to be an alive chicken, so there is a bit of a difference. :)
I think it's similar to the "Katze vs. Kater" issue, where "cat = Katze", but technically "Katze" might mean "female cat" (alhough there's "Kätzin = female cat" as well) as opposed to "Kater = male cat", but as long as you don't know what it is, you call it "Katze".
...Only you can tell apart female and male chickens easier, so you'd be more likely to call a male one "Hahn" right away, but the general term (which you also find on restaurant menus) is still "Huhn".
And if I see at first glance that the chicken riding on the bus is male and also happens to be young/small/cute, I don't think I'd actually call it "Hähnchen" instead of "Hahn", because a "Hähnchen" usually means a fried chicken. (Or if I do, I'd feel I need to add that it's an alive one.) It's a specific issue with "Hahn/Hähnchen", of course, that you don't get with e.g. "Kätzchen" (which could be a kitten / small cat, but also could be an adult cat I happen to find cute).
"im" is short for "in dem". (And "zum" is short for "zu dem".)
"der Bus" is masculine, so it's not "in der Bus", but "in dem Bus", thus you can shorten it to "im Bus".
Neuter words have the same dative case: "in dem Haus" can be shortened to "im Haus".
Feminine and plural words don't work: "die Küche" - "in der Küche"; "die Wälder" - "in den Wäldern"