
Samasboy1 |

Going strictly by RAW, the Evolved Familiar feat actually specifies that familiars cannot take the Mount evolution.
Of course, this is do to the assumption that no familiar will be at least on size larger than its master.
Since a mailer familiar WOULD be one size bigger than a small master (at least in battle form) it seems reasonable to allow it.
Or if the master was tiny/diminutive with a small/tiny familiar respectively.

Entryhazard |

Going strictly by RAW, the Evolved Familiar feat actually specifies that familiars cannot take the Mount evolution.
It does exclude familiar by the assumption that they cannot be larger than small.
The familiar must conform to any limitations of the evolution.
And Mauler in battle form does if the master is small. And a familiar can stay in battle form as long as it wants.
For instance, no familiars can benefit from the mount evolution
Benefit=/=Take

Mark Hoover |

Don't worry about mount; you can ride a medium sized Comp. already. Just hop on its back and use your evolution for increasing it's damage die, adding a tail or claw attack, or possibly tossing in Bleed. Consider:
The comp has a bite. You add claws; it's got 3 primary natural attacks. You take another 1pt evo at 5th level - now it's got pounce. Now your compy, with a handy Bull's Str is racing to the frontline and attacking with claw/claw/bite for a potential 2d4/1d6 with at least +5 per claw and a +7 on the bite.
Take the wizard arch spirit binder and give your familiar full bab. With Str increases from 5th level mauler, battle form and a Bull's Str spell it's got a +10 at least to hit, possibly higher. Hitting w/all 3 attacks delivers 25 damage on average.
Over the course of your adventuring career you could be granting your familiar the following through evolutions:
Bleed - +1d6 bleed damage from one attack (can be added to multiple attacks; doesn't stack)
Claws - 2 1d4 primary attacks
Climb speed
Hooves - 2 1d4 secondary attacks
Improved Damage - increase the damage die for one attack (can be taken multiple times, affecting a different attack each time)
Improved Natural Armor - +2 NA (can be taken once every 5 levels, adding +2 ea time)
Pincers - 2 1d6 secondary attacks
Pounce
Reach on one of its attacks (can be taken for multiple attack types)
Slam - a 1d8 primary attack
On top of other stuff. Heck just taking Claws, Bleed, Improved Damage - claws and Improved Natural Armor gives you a claw/claw/bite monster that's dealing 3d6 plus 1d6 Bleed with a natural AC of +5 at 5th level. At this point the spellcaster is just around to buff and protect your familiar.