Not by RAW
Simply because the rules written, which you have quoted, say nothing about it being such. It doesn’t mention Leadership or Leadership’s cohort at all, and while it mentions similarities between Wild Cohort and Animal Companion, it does not actually state that Wild Cohort is or counts as Animal Companion.
It’s more than reasonable to houserule it, though
Allowing someone to qualify as having an Animal Companion or Cohort, because they have the Wild Cohort feat, is perfectly reasonable. I have done so in many games, and it’s never been an issue.
In this case, though, I would not actually state any interaction between these things. If you are a druid or ranger and take Wild Cohort, you just get both an Animal Companion and a Wild Cohort, as separate creatures. Same, too, if you take Wild Cohort as well as Leadership: you get a regular cohort as well as the wild one. The two don’t interact with one another.
You can't.
The second type of bond allows a paladin to gain the service of an unusually intelligent, strong, and loyal steed to serve her in her crusade against evil. This mount is usually a heavy horse (for a Medium paladin) or a pony (for a Small paladin), although more exotic mounts, such as a boar, camel, or dog are also suitable. This mount functions as a druid's animal companion, using the paladin's level as her effective druid level. Bonded mounts have an Intelligence of at least 6.
The enhancement at level 11 adds the celestial creature simple template and "becomes a magical beast for the purposes of determining which spells affect it."
The Monstrous Mount feat can give you something a little cooler than this, but of the mounts listed, unicorn is not one. The options listed there are:
- Griffon
- Hippogriff
- Hippocampus
- Worg
Monstrous Companion also gives you access to a companion which is monstrous, but again unicorn is not listed there.
Leadership can grant you a cohort in addition to your animal companion, but doesn't modify your divine bond class feature .
This does not mean you can pick a celestial creature or magical beast, only that the bond you already have (since level 5 when you chose to take a steed) becomes a magical beast.
Without GM fiat, there is no way to gain a unicorn as your divine bond mount (and I think most unicorns would be quite offended by the notion that they are mere beasts of burden, to be mounted and ridden upon by men, for noble purposes or otherwise!).
Best Answer
The feat allows you to combine Leadership with animal companions
Effective cohort level is used to define the level of your Monster Cohorts (Leadership) and here it helps to calculate your druid level on the Animal Companion table for monster cohorts. This effectively reduces your druid level for the purposes of calculating feats, skills, bonus tricks and special abilities of your companion. This is explained under the feat Monstrous Companion (published on Cohorts&Companions Player Companion):
In other words, those companions are stronger than standard companions (animals), and to balance that out, they gain fewer benefits from your effective druid levels. Keep in mind that non-druids can take this feat (rangers, cavaliers, hunters, etc), so effective druid level is that class's level as a druid, which may or may not be equal to their class levels (rangers are level -3 for instance).
To help explain it, this is the text from the book (page 9) right before Monstrous Companion:
Monster Cohorts
Not many people know, but the GM may allow a character with Leadership to take Monster Cohorts (Bestiary, page 316). But this reduces the cohort's effective cohort level. Meaning that a Ghoul cohort (level 5 on the table) is harder to upgrade than a human npc, which would start with 5 class levels instead of none.
Monster cohorts can take one class level per level difference between your Cohort Level and the companion's Effective Cohort Level. And allowing the character to take a monstrous companion and an animal companion could easily unbalance the game, so the feat allows you to combine both into a single creature with class levels and animal companion bonuses.
Dragonnes
As explained in the bestiary entry for Dragonnes (bestiary 3, p. 104):
So, dragonnes are level 10. Meaning that you must have a Leadership Score of 14+, and it would have to be a 17th level druid to be able to keep one as both your cohort and your companion, as illustrated under Monstrous Companion:
And for every effective druid level above 17th, your dragonne would gain 1 class level on the Animal Companion table:
Alright, but what's the difference?
Leadership will allow you to take a dragonne as cohort, but you must have a Leadership Score of 14 to have a level 0 dragonne. This can be obtained at 14th level by a druid with +0 charisma, maybe earlier, maybe later, depends on your score really. A 17th level druid could have a dragonne with 2 fighter levels, for instance.
Monstrous Companion is an alternative, and you can't have both. A dragonne companion can be taken by a 17th level druid, and would have an effective animal companion level of 0, gaining one animal companion level per druid level above 17th.
If instead of a dragonne (10th), this 17th level druid takes a pegasus (6th), this pegasus would gain 6 class levels with Leadership, or 4 animal companion levels with Monstrous Companion.
But keep in mind that the GM has the final say on whether Leadership is allowed in his game. A druid that takes Leadership will have two "pets" to control, for a total of three characters. While Monstrous Companion is easier to balance out, since its a single creature.