
Teupokoles |
If it doesn't conflict with the theme to have dinosaur companions, then check out the Ankylosaurus.
Unless somethings been added since i last played druid, it has the highest natural AC of any companion, it gets large for reach, uses a single big natural attack which is easier to improve with evolved companion and the stun it delivers is just crazy good.
15ft reach, combat reflexes and stun on its attacks makes it great for intercepting charges and keeping them stuck too far from you for them to fix it with a 5ft-step.
The Big Cat is an obvious choice, pounce is well hard, but it doesn't exactly aid in derense. To best use the Tiger it needs to charge right away, whereas something with big reach can better defend you, or even move into melee while still keeping you inside its reach.

ErichAD |

Mokele Mbembe Is an excellent defensive choice due to having reach at early levels extending naturally to 15ft reach at 9th level when their size increases. Make that 20 with evolved companion and you have some good defense out there.

doomman47 |
If it doesn't conflict with the theme to have dinosaur companions, then check out the Ankylosaurus.
Unless somethings been added since i last played druid, it has the highest natural AC of any companion, it gets large for reach, uses a single big natural attack which is easier to improve with evolved companion and the stun it delivers is just crazy good.
15ft reach, combat reflexes and stun on its attacks makes it great for intercepting charges and keeping them stuck too far from you for them to fix it with a 5ft-step.
The Big Cat is an obvious choice, pounce is well hard, but it doesn't exactly aid in derense. To best use the Tiger it needs to charge right away, whereas something with big reach can better defend you, or even move into melee while still keeping you inside its reach.
How are you getting 15 foot reach? An ankylosaurs should only have 5 foot reach at large size due to being quadrupedal.

avr |

Vampiric companion says it gains the undead type. What does this mean?
An undead creature possesses the following traits (unless otherwise noted in a creature’s entry).
* No Constitution score. Undead use their Charisma score in place of their Constitution score when calculating hit points, Fortitude saves, and any special ability that relies on Constitution (such as when calculating a breath weapon’s DC).
* Darkvision 60 feet.
* Immunity to all mind-affecting effects (charms, compulsions, morale effects, patterns, and phantasms).
* Immunity to bleed, death effects, disease, paralysis, poison, sleep effects, and stunning.
* Not subject to nonlethal damage, ability drain, or energy drain.
* Immune to damage to its physical ability scores (Constitution, Dexterity, and Strength), as well as to exhaustion and fatigue effects.
* Cannot heal damage on its own if it has no Intelligence score, although it can be healed. Negative energy (such as an inflict spell) can heal undead creatures. The fast healing special quality works regardless of the creature’s Intelligence score.
* Immunity to any effect that requires a Fortitude save (unless the effect also works on objects or is harmless).
* Not at risk of death from massive damage, but is immediately destroyed when reduced to 0 hit points.
* Not affected by raise dead and reincarnate spells or abilities. Resurrection and true resurrection can affect undead creatures. These spells turn undead creatures back into the living creatures they were before becoming undead.
* Proficient with its natural weapons, all simple weapons, and any weapons mentioned in its entry.
* Proficient with whatever type of armor (light, medium, or heavy) it is described as wearing, as well as all lighter types. Undead not indicated as wearing armor are not proficient with armor. Undead are proficient with shields if they are proficient with any form of armor.
* Undead do not breathe, eat, or sleep.
Basically a whole bunch of immunities but your companion becomes made of tissue paper as far as hit points go.

doomman47 |
Vampiric companion says it gains the undead type. What does this mean?
Undead type wrote:Basically a whole bunch of immunities but your companion becomes made of tissue paper as far as hit points go.An undead creature possesses the following traits (unless otherwise noted in a creature’s entry).
* No Constitution score. Undead use their Charisma score in place of their Constitution score when calculating hit points, Fortitude saves, and any special ability that relies on Constitution (such as when calculating a breath weapon’s DC).
* Darkvision 60 feet.
* Immunity to all mind-affecting effects (charms, compulsions, morale effects, patterns, and phantasms).
* Immunity to bleed, death effects, disease, paralysis, poison, sleep effects, and stunning.
* Not subject to nonlethal damage, ability drain, or energy drain.
* Immune to damage to its physical ability scores (Constitution, Dexterity, and Strength), as well as to exhaustion and fatigue effects.
* Cannot heal damage on its own if it has no Intelligence score, although it can be healed. Negative energy (such as an inflict spell) can heal undead creatures. The fast healing special quality works regardless of the creature’s Intelligence score.
* Immunity to any effect that requires a Fortitude save (unless the effect also works on objects or is harmless).
* Not at risk of death from massive damage, but is immediately destroyed when reduced to 0 hit points.
* Not affected by raise dead and reincarnate spells or abilities. Resurrection and true resurrection can affect undead creatures. These spells turn undead creatures back into the living creatures they were before becoming undead.
* Proficient with its natural weapons, all simple weapons, and any weapons mentioned in its entry.
* Proficient with whatever type of armor (light, medium, or heavy) it is described as wearing, as well as all lighter types. Undead not indicated as wearing armor are not proficient with armor. Undead are proficient with shields if they are proficient with any form of armor.
* Undead do not breathe, eat, or sleep.
Hmm so any possibility I could make them a construct instead?

avr |

There's the augmented companion archetype which gives a couple of such abilities but not the full type. It's not that great but it's better than losing 3+ hp/HD like vampiric companion would. I don't know of a way to get an actual construct as your animal companion, though there are options for a familiar.
BTW, if you want a survivable companion and are less concerned with its damage output then the beast speaker (sun falcon) feat and vampiric companion would work together well.