How does using a warhorse as a beast companion work?

Question

I'm creating a Human Ranger Beast Master (D&D 4e) and I would like to have a Warhorse beast companion and use it as a mount. And I have several questions concerning this:

1. If I buy a warhorse and "train" him to be my beast companion, does he lose his trample attack and charger mount trait? And will his ability and defense stats change?
2. What will the carrying load be of my warhorse if his ability stats change? Because if a beast companions stats change, and loses his normal load from a horse, his carrying load will be 14*10 and will not be able to carry me.
3. Do I need the feat beast rider to ride my warhorse?
4. Does the Mark of Handling speed and AC bonus stack?

Thanks in advance.

Edit I found Dragon Magazine 392 which gives your the horse beast companion.

Horse

Ranging from ponies to warhorses, the horse beast companion is most effective when ridden.

Ability Scores: Strength: 14 Constitution: 14 Dexterity: 16 Intelligence: 6 Wisdom: 12 Charisma: 6

Size: Medium or Large (you choose at creation, but must be one size larger than you if you want to use the Mount ability). Speed: 10 squares Defenses: Armor Class: 14 + level Fortitude: 13 + level Reflex: 12 + level Will: 12 + level Hit Points: 14 + 8 per level Attack Bonus: Level + 4 Damage: 1d8 Melee Basic Attack: Hooves Level + 4 vs. AC 1d8 + Strength modifier damage. Trained Skill: Endurance

These abilities are significantly less then the warhorse and make it a bad choice in heroic tier. I would rather have a normal warhorse mount than a horse beast companion.

What do you guys think of changing the warhorses HP + Defences and ability stats to that of a horse beast companion, and changing the trample to an encounter power? And perhaps the Charger Mount Trait to apply to friendly riders of lvl 5 or higher?

Answer 1

You can not have a Warhorse as beast companion.

Beast Mastery: You gain a beast companion, chosen from one of these categories: bear, boar, cat, lizard, raptor, serpent, spider, wolf or horse. (Martial Power page 38 + Dragon 392)

A Horse Beast Companion is not a Warhorse in any way concering rules. Your DM could houserule a Warhorse is a horse, but I would advise against it. It would be overpowered, as a Warhorse is quite more powerful than any of the allowed types. (HP, trample, defenses, attack, damage)

The horse as a Beast Companion is weaker than a Warhorse for balance reasons. Changing HP and Defenses is not enough, change damage too, for trample as well.

Trample and Charger could be added at the cost of one feat each if they are usable at-will, or both for one feat if usable once per encounter. Still, trample should do 1d8 damage.

I would give you a free Saddle of Strength, it takes up the slot of the much better Impenetrable Barding anyway.

You need the Beast Rider feat for it just like with any other Beast Companion. For this reason and because of the better feat support, I would still go with a refluffed Bear.

Mark of Handling gives feat bonus to mounts, and feat bonus to beast companions. Feat bonuses do not stack, so no, it is not +4 speed +2 AC.