Why? I understand mastodons but why leopards?
Leopards are quite good, at least on paper, as well. They are among the few pets with 2 additional, 3 total attacks, only smilodons get more. However leopards compared to smilodons:
- get better AC, eventually by a good few points,
- can trip enemies with main attack which is always nice as it can provide with AoOs for everyone within reach,
- after a few levels they should be using DEX for both attack and damage rolls (I think). And of curse DEX is also AC. This is handy when you want to buff them,
- and finally they end as medium sized instead of large. If you use several pets in addition to some characters on the frontline it might be a plus. Supposedly it can get too crowded while using several large pets in some situations.
However smilodons still get 4 additional attacks instead of "only" 2 and can't be beaten in terms of raw damage output so I wouldn't say that they are "noob choice". They're THE choice if raw damage potential is what you are looking for in a pet. If you want more AC and want the pet to contribute something to the party other than their own damage then leopard becomes very good. Personally I'd probably diversify if I had more than 1 pet in party. 1 smilodon + 1 leopard for example. But that's just me, not some objective truth. I'd like some diversity in my zoo.
Dead pet does 0 damage. I will leave it at that.
Most people seem to fail to realize that Leopard do not have Power attack and all other animal companion do. As for AC all animal companion wind up at a very similar AC number, they are all within 3 AC of each other 49 is the highest(Leopard and mastdon) and 46 is the lowest (smilodon and various others) at level 16. I think a wolf is 48 and dog 47. Also stats wise all the companions wind up only +1 or +2 different from each other. A leopards dex is only +1 or +2 different than strength on a smilodon, even more the strength on the Leopard itself is only a bit different than its dex. The fact that it traded weapon finesse for power attack was actually kind of dumb since, IIRC, they wind wind up with 28 str versus 32 dex but lose power attack for that +2 and take a net -6 on damage. And the smilodon winds up with 30 or 32 str anyway. Similarly the smilodon just winds up with 28 dex versus the Leopard 32 dex and is only +2 difference as well with the +1 size creaing the +3 AC difference. Also the lower str on leopard makes it somewhat worse at tripping than a dog and a dragon polymorphed dog does more damage anyway due to power attack, while only have I think 2 more ac.
IMO the AC differences between pets are not that substantive. The damage difference can be gigantic and special effects are substantive. The vast majority of companion AC and stats come from levels and have little to do with their type. A level 12 smildon is, IIRC, 39 AC and a level 16 is 46 AC. People seem to have taken the stats shown at the level 7 change and thought those meant something, but in reality levels even everything out to within 1,2, or 3 on most things. The main difference are feats and special attacks. Not having power attack puts Leopards in an entirely different class vs all other animal compantions and their weapon finesse is, frankly, not an advantage at all in fact Mastadon's strength is either the same or +1 better than Leopard dex. There are really only two main considerations do you want trip or damage? If you want both you can get both by NOT choosing Leopard for trip but you have to wait for level 6 arcane spells. So in general just either chose between damage (which means smilodon) or trip (Leopard is fine for this, but so are the others). But any companion can tank when properly buffed, they can all get over 60AC.
In reality Leopards don't necessarily do more damage than Mastodons with the 3 attacks versus 2 attacks, because they lack power attack and both are actually similar in damage, but the Leopard is inferior in dragon polymorph unless you really want trip in which case go with dog, since dog has better CMB and power attack.