Malanis' Comments

The comment stating they spawn at midnight on pacific time seems to be accurate, at least for Whisperwind (2 a.m. server time was pacific time, spawned within moments of stricking 2).

There are two seperate areas called the Master's Cellar. They spawn in both, but I found a great deal more on the 'cave side'. The 'cave side' is in the house ruins closest to the summoning stone.

No secondary pets spawn. So a very easy pet to capture for someone just starting out. 

I tried farming my rare in Gorgrond. No luck today besides an uncommon.

I battled a few Gold Beetles and Parched Lizards, but the Brown Marmot didn't show up as a secondary at any point. (For now) it seems the Brown Marmot doesn't appear as a secondary to any pet besides a Brown Marmot.