The hench desired by the collector has a variant formula. This means that it can only be produced through mixing and its formula is changed periodically.
* The mission can only be done 1 time until the next round. There is no point in making several monsters, in the end, the collector will only accept the monsters mixed with the new formulas.

This time hench is: Foxy Dog (Lv.120)

The first formula discovered for this hench is: 
DevilDusty (with mix stage item 2) + Seki-Shu-Ryu (with mix stage item 2)

The current reward is: 

1x [Curesed Mark] / 1x [Olympus Soul Potion] / 1x [Dark Cloud]

Find the collector at Mekrita.

Remember: This formula and reward can change periodically.