My cleric5/alienist1 picked up Leadership at level 6 and got a Darnu (from Denizens of Avandu) as a cohort. It's a magical beast that can speak and understand any language and looks like a medium sized dog. It's an ECL 4 so it worked out well for a 6th level character. When I hit level 7 I assume I just add a level of magical beast and do that everytime I level, right? It states that after so many hit dice (I think 7) it becomes size large. Is the a level adjustment for becoming size large and if so do you earn that level first then increase in size or increase in size and spend two levels for it?
To expand on the previous post, most racial HD have an implicit "Level Adjustment Buoyancy". Fey HD are pathetic; a fey with more than 3 racial hit die should have some nice abilities/ability scores and a low level adjustment. Conversely, dragon HD are incredible, and are almost equivalent to a PC level, and as such any "extra" abilities will cost you much level adjustment.
Magic beast is about baseline, not good, not bad. So getting to go up to large size is kind of like the wizard getting access to a new tier of spells after a few levels. Its paid for with the HD you already took.
I'm planning on figuring out the "bouyancy" ratio's of the different HD relative to level adjustment, to see if there is a way to A: recalculate some of the level adjustment issues in the current LA rules, B: further fine tune my own personal LA rules, and C: see if its possible to take certain creatures with excessive racial HD and cut them down to single HD with an appropriately modified level adjustment.
Yep, fey and construct are on the bottom of the list, and outsider and dragon are at the top. The problem I have to deal with is that there is also the effect of the type as well as the type's HD. Fey get crap, but constructs have a slew of perks. Same with undead and elementals. But humanoids, animals, and monstrous humanoids are entirely dependant upon the specific creature for perks. Dragons get a few perks, and outsiders almost get a negative for the issues of raising them from the dead.
Ending stream of thought post now.
Along these lines, if a PC takes the Dragon Cohort feat (from Draconomicon), how would the cohort advance as the PC gains XP and levels?
First thing you have to decide is relevant to your World Versimilitude: Can dragons "age" faster than "normal"? Specifically, can a dragon gain its racial HD at a faster rate than what is indicated by the Age Category and Years chart? You have to decide if yes or no. If you need a justification for "yes", you might say "adventuring" dragons are sleeping less, being far more active, and putting their minds in real situations and scenarios rather than just mentally fabricating them while they lounge about the lair. Due to this higher rate of activity, and thus, food intake, they grow faster. Maybe there is a cultural/religious reason why most dragons "take it slow". Maybe not.
Now then, if you decided they can gain racial HD faster than normal, here is your options: Less work or more work.
Personally, I would use the Savage Species style breakdowns for dragons that were given in a pair of Dragon Magazines (cant recall which off the top of my head by number, but the metallics are in the one with a dhite dragon on a black background on the cover, the chromatics in one with a dragonslayer about to huck a spear at a red dragon).
Otherwise, you will need to make the breakdowns yourself. Consider the rate at which the dragon gains natural armor, breath weapon improvements, and ability score improvements at each age category (and spells, DR, SR, and fear aura, down the line). Spread them out decently well over a few levels, and make a note of when the actual Level Adjustment goes up, if it does.
Now, if you decided your dragons don't gain racial HD any faster, or you don't have those Dragon Magazines and don't want to do all that work, you could just have a dragon advance in PC levels. Depending on what books you have access to, this can give the dragon a wide variety of choices. WARNING: very easy for a PC classed dragon cohort to outshine his "boss" and by proxy, other party members.
You may also want to consider, if you run cohorts in things like adventure paths and such, where the gear is pregenerated for a certain party size, that you can often forego the level adjustment and have the dragon cohort go "au naturale". The extra 3-4 hit dice offset the lack of gear, and it makes for much less bookkeeping and scene stealing, especially at higher levels. I would mention that this is something I wouldn't do with a dragon with PC levels, as the varying levels of Multiple Ability Dependancy could make some versions overpowered, or others quite weak. Gearless cohorts work best with creatures who can stay in a competitive racial HD advancement, like dragons, outsiders, etc.
Thanks for your well-thought-out advice! Another thing I wasn't sure on was whether a dragon cohort would arrive with any gear. A normal cohort would, and a normal dragon would have hoarded some treasure, but it's not clear to me how things would work for a dragon cohort...
A cohort is a non-player character, and as such is a "NPC" rather than a "monster". It should have NPC gear appriopriate for its ECL.
The dragon that decides to follow some mortal around is already a bit off in the head, so its not a big deal for it to trade all of it's shiny gold for things that will help it live longer. Or, look at it in the sense that having its hoard as gear means it never has to worry about where it's hoard is.