The corpse rider class feature seems to stipulate you can raise anything quadrupedal of large or medium size as a mount
-If the undead mount ability is selected, the seal-breaker gains the ability to reanimate any Large corpse (or a Medium corpse for Small seal-breakers) as his corpse mount.-
-The corpse animates as a quadruped of the same size that the seal-breaker can ride, regardless of the corpse's living form.-
It does not say if said quadruped remains able to fly and use spell like ability so does it mean i could just beat the crap out of a living dragon of large size and or an unicorn and take it as my mount totaly ignoring the use of feats such as ''monstruous mount'' and ''monstruous companion''?
Sure it says ''same base statistic as a horse etc'' but does it goes as far as flight speed and supernatural ability?
Having no special movement types is part of a horse's "base statistics", so I'd say no. The archetype really does not seem to want your undead mount to be stronger than a regular Antipaladin mount, which is unfortunate. It would be better if you just applied the skeleton or zombie template on your raised mount, and had the abilities it can retain scale up with Antipaladin level like polymorph spells.