Dragomir's Comments

Definitely not found only as a secondary pet.  There are quite a few of these as primary pets in the Bleeding Hollow Hunting Grounds (Tanaan Jungle 27, 55).

I found my rare frog as secondary to a maggot directly southwest of the "F" in Forestsong, in the demon camp near the pool of water.

I got my rare as a secondary to Alpine Chipmunk just south of Mazthoril.  Alpine Hares, Alpine Chipmunks, and Crystal Spiders can all have Rabid Nut Varmint 5000 as a secondary.

These are primary pets on top of cliffs.  You can access them by hitching a ride on a Highwind Albatross. Go to the north shore of Mossgreen lake, hit a Highwind Albatross with a ranged move and stop autoattacking.  It should pick you up and start carrying you.  Once you're over a pillar with moths on it, start attacking, it will stop, and, after you kill it, drop you on the pillar.

The rare I found was level 23 after being captured, has 1371 hp, 254 attack, and 254 speed.