the best thing that the pal machine pops out are the ripper/resist dragon,these are dragon which come with a fixed damage skill and a leader skill which cuts in 30% the damage of their kind received and by 50% when fully evolved,they make decent filler while waiting for better monster... so far i have the earth one,the water one and the light one
then it can also pops devil,these are guys who completly suck at everything except attack,their attack is amazing and can reach godlike level
supposedly (but i never seen it happen so far) it can also pops out golem (huge hp but meh everything else) the healer girls (a bunch of monster with healing capability and high recover... the red one has a skill that delays enemy attack and the blue one has a leader skill that give you back tons of HP when fully evolved and the black one has the same skill as neptune but suckier stat so she can work as a gimped version of neptune) and the red and black heart converter (decent all rounder,has a skill that turns heart into their own color... the valkyrie i was mentioning earlier is the light heart converter) one of the reason people says that red and black are easier to build IS because the red and black heart converter can be gotten from something that isn't the rare egg machine or some super hard optional dungeon
lastly during event (like right now) there is a chance it will give you pengdra or metal dragon,these suck but give tons of XP when feed... maxing a pengdra only takes 2,400 XP but feeding it to a monster give 6,000 back (9,000 if same color) if you evolve the pengdra then you get a whopping 30,000 XP per penguin
metal dragon also appear during specific time of the day for one single hour,there is really no way to know when other than to check that fansite creepy posted a while ago