Because you did a toadskin map. If you didn't notice, the treasure chest was silver-lined instead of golden like on boarskin and peisteskin.
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I'm also inclined to believe it's totally random and does not depend on your party size.
We've been running peisteskin maps with a 8-man group consistently, and while there might be a correlation between a type of monster that spawns and the reward, there is certainly none between it and the location/party size.
Oh, and before I forget - a giant tortoise (not sure if it was territorial or alpha) dropped a Bluebird pet once. No idea if this can be replicated as pet rewards seems to be kinda rare.
Ok well I really don't pay that much attention to color of the chests all that much ;/
Just to add to the record, I ran a Peisteskin Map solo as SMN; here's what I got:
5 Tomestones of Mythology
5 Mega-potions of Strength
100 Earth Shards
1 Darksteel Nugget
2 Spruce Logs
1 Hippogryph Skin
1,692 gil