I think Lord Pendragon has covered most elements, but I will toss in a couple of others:
1) The DMG discusses combining Special Mounts with Cohorts as an option. That is as close as the core rules come to what you this topic is about.
2) The creatures that are suitable as familiars are on the same list as the core considers "not suitable for cohorts", which is one reason why the topic is not covered.
3) Cohorts are NPCs whose loyalty can vary over time; they *can* generally be bought off, blackmailed, charmed, or otherwise "persuaded" to betray their master. They can also become tired of being the errand-runner, and just up an leave (i.e., their level gets too close the master's). Familiars are nothing like this, and the mixture of their incorruptible loyalty with the increasing powers of a cohort makes the spellcaster potentially too potent.
All that being said, what you may want to consider is the notion of Improved Familiar. It is, like Leadership, a feat that the DM must decide to introduce. Most improved Familiars, though, are a little tougher in combat than the average familiar. See the DMG, pages 200-204. Adjacent sections also discuss using Leadership to attract a cohort that can serve as a Special Mount, which is the only combination the Core rules address.
If the selection of Improved Familiars in the DMG is not quite up to what you need, there are more in Complete Warrior that are specifically geared for a more combat-oriented role.
1) The DMG discusses combining Special Mounts with Cohorts as an option. That is as close as the core rules come to what you this topic is about.
2) The creatures that are suitable as familiars are on the same list as the core considers "not suitable for cohorts", which is one reason why the topic is not covered.
3) Cohorts are NPCs whose loyalty can vary over time; they *can* generally be bought off, blackmailed, charmed, or otherwise "persuaded" to betray their master. They can also become tired of being the errand-runner, and just up an leave (i.e., their level gets too close the master's). Familiars are nothing like this, and the mixture of their incorruptible loyalty with the increasing powers of a cohort makes the spellcaster potentially too potent.
All that being said, what you may want to consider is the notion of Improved Familiar. It is, like Leadership, a feat that the DM must decide to introduce. Most improved Familiars, though, are a little tougher in combat than the average familiar. See the DMG, pages 200-204. Adjacent sections also discuss using Leadership to attract a cohort that can serve as a Special Mount, which is the only combination the Core rules address.
If the selection of Improved Familiars in the DMG is not quite up to what you need, there are more in Complete Warrior that are specifically geared for a more combat-oriented role.