Do monstrous scorpions grow in size as they age, and if so at what rate?

Question

The Arms and Equipment Guide lists the purchase price for a monstrous scorpion egg at 1,000 gp and the training cost at 2,000 gp. It also mentions how medium sized creatures require a huge size mount rather than a large, because of how low the scorpion rides to the ground. However, based on the text I am not sure if it is saying that you must purchase a huge size specimen or if you must wait until your large size specimen reaches huge size.

Are the various sizes of monstrous scorpion one species across different age categories, similar to dragons? Or is it necessary to purchase a huge scorpion egg, hire an expert trainer, and leave the egg with them until it hatches?

Answer 1

The sizes are likely not age categories

The monster manual unfortunately is extremely sparse on any kind of ecology and lore information for these monsters. However, based on the text in the Arms & Equipment Guide, it appears that the different size categories are not different age classes of monstrous scorpion, but different subtypes. The Arms & Equipment Guide says:

Humans and their ilk find Huge or larger scorpions to be better fits. However, this size of vermin is extraordinarily difficult to tame in adulthood, and the consequences of failure are dire.

The sentence about how hard it is to train an adult huge scorpion implies that huge scorpions of different age categories exist. If the size was a function of age category of the scorpion, then this would not be the case, younger than adult monstrous scorpions would not be huge scorpions.

The entry continues:

It is practically a necessity to purchase a scorpion from a reputable trainer rather than attempt to train it alone.

So, in answer to your further questions, the trainer would need an egg from a huge scorpion variant, breed and train it for you, and you then would need to buy the trained scorpion. However, no price or carrying capacity stats for such a huge scorpion are listed. So, if you plan on using such a scorpion as your mount, the DM will have to make up something reasonable.

You likely need a trained rider, too

Note that the lore for vermin mounts also declares:

Vermin serving in the role of domesticated creatures is such an alien concept that vermin riders are either extraordinary individuals or part of an unusual society that uses vermin in everyday life. (...) Vermin riders must remain with their mounts almost constantly to maintain their bond. Mount and rider even sleep in the same room in specially constructed barracks/stables. As a result, the bond is deep: Riders grow close to their mounts and connect with their subtle personalities.

All of this strongly suggests that such a vermin mount is not something you just pick up at the exotic stables down the road. You need to have a deep, long-nurtured bond with the vermin to be able to use it as your mount and control its base instincts -- not only the mount needs to be trained, also the rider needs to understand how to ride such a mount. This might be something better suited to a character concept (maybe, using a paladin or ranger type that has a special relationship with their chosen companion or mount), than something you just buy and use, and for that, it probably will be best to work with the DM anyways.

Answer 2

Contra @groodythehobgoblin's answer, the size categories probably should act as ersatz age categories.

You tagged this dnd-3.5e, so I'm using that monster manual. The 3.0 manual had different numbers, but a similar relationship.

What we are looking for ideally is an entry for "Monstrous Scorpion" with an advancement entry that says "1 HD (Tiny), 2-3 HD (Small)..." etc. That's missing, so this is not a purely read-and-interpret answer.

What we actually have is a bunch of entries that read "Monstrous Scorpion, [Size]". The advancement for each only lists a hit die range (and only for the larger ones), not a size change. Read strictly, that suggests that the different sizes are totally distinct species of scorpion. Every "Monstrous Scorpion, Huge" pops into existence fully-formed as a huge Vermin with 11HD. If that is correct, you're good to go already: the egg is probably unpleasantly large for transport, but as soon as the immature scorpion hatches, it's ready for training and riding.

However, I note that breaking the sizes into different monster entries is far easier to use as a DM, which may be why WotC chose to present it this way. I also note that the size ranges don't overlap. So, stacking them up, we get:

• 1/2HD: Tiny
• 1HD: Small
• 2-4HD: Medium
• 5-9HD: Large
• 10-19HD: Huge
• 20-39HD: Gargantuan
• 40-60HD: Colossal

...which is a remarkably clean progression. Were I running your game, I would use that progression as though it were a single monster with that in the Advancement block.

(In fairness, the progression is not quite as clean for other, similar vermin. E.g. treating the centipede like this means it's missing an entry at 2HD).

However, there is also another piece you're going to run into pretty fast: how do Vermin advance? Nominally every 4 HD adds 1 CR, which suggests that your scorpion needs to be gaining HD four times as fast as you are gaining levels if it is going to keep up with you. I have no advice on how to a handle that other than talking to your DM.