Serpentine Dragons are the supreme Magery pets, as far as spellcasting ability goes. They spawn with up to 1,040 INT (way more than any other pet, and well above 700 Cap), which gives them slightly more spell damage (a lot more with Mind Blast, which they cast with Magery Mastery), as well as a higher mana regeneration rate. However, even with 1,040 INT/1,500 Mana/30 MR/120 Med/120 Focus, a Serpentine Dragon will eventually run out of Mana in a long enough fight, and be forced to rely mainly on melee damage, which they're not the best at. Due to their high INT/DEX, Serpentine Dragons max out around 580-590 STR, and they're very cramped on Training Points, so you can't really fit 120 Tact/Anat on them, nor fit HPR/SR. This is without even touching their base 480 HP.
Even with all this magical power, they are still out damaged by a Chiv+AI pet in melee range. Not to mention all the various bugs with Magery Mastery.