Sell it. The rewards really aren't very worthwhile, at least on Leviathan. Sure you could get the pet supposedly, but that's equally as big of a gamble as the sapling/tonberry/bird is. I've done 2 maps, the first yielded around 3/4 of a return from potentially selling it, so not bad. The second however yielded 1/100 of a return... so it's equally as much of a gamble as the normal maps.