During lent a Roman Catholic may not consume meat other than that of a fish. However, there are some non-fish animals that have been considered okay to consume during that time, like beaver for example. What other non-fish animals are okay for a Roman Catholic to eat during lent?
Abstinence from meat, or from some other food as determined by the Episcopal Conference, is to be observed on all Fridays [...] (emphasis mine).
In other words it is to be determined by the bishops exactly what constitutes abstinence. The bishops of Quebec can simply state that beaver meat is OK to eat on days of abstinence. They don't have to declare it 'fish'. I would imagine that this is common practice in areas where fish is very hard to come by and meat is a staple food. In areas where meat is a rarity, giving it up would be little sacrifice and the rules might again be changed. As this page puts it, "when we abstain, it's not because the food is impure; we're voluntarily giving up something good, for our spiritual benefit". The exact nature of what is given up is of secondary importance.
As DJClayworth wrote, nowadays it's not a strict rule what you can eat in lent. But in Middle Ages, there were strict rules about it. All fish were allowed. But fish were not defined as today, but as "all water animals", including a beaver or a capybara. Molluscs were permitted too - thats why monks (some order had to fast for most of the year as others fasted in lent) brought Roman snails (helix pomatia) to some regions where it's not native.
The classical definition of what constituted "fish" in the days of St. Thomas Aquinas was based on the Summa Theologica's understanding of animals which took into account both animal habits as well as anatomy.
Usually seafood was permitted and occasionally cold-blooded animals might be acceptable in certain areas. First Things offers a great list of some weird Lenten foods.