Strictly speaking your mount can't be targeted with a Reduce Person spell. However, they are eligable for Reduce Animal. Reduce Animal does not state that it can be made permanent, but since it's based on a spell that can be made permanent your DM may allow you to do this, but you'll need to check with them first. It will probably cost more than a permanent reduce person spell because reduce animal is higher level.
Assuming you can make it permanent its worth noting that because the spell is not a polymorph effect it does whatever the spell states it does, nothing more and nothing less.
The important bits that would apply to your mount are as follows.
The target gains a +2 size bonus to Dexterity, a –2 size penalty to Strength (to a minimum of 1), and a +1 bonus on attack rolls and AC due to its reduced size.
A Large humanoid creature whose size decreases to Medium has a space of 5 feet and a natural reach of 5 feet. This spell doesn't change the target's speed.
All equipment worn or carried by a creature is similarly reduced by the spell.
Also, because it's Reduce Animal
Reduce the damage dealt by the animal’s natural attacks as appropriate for its new size
So, it would become Medium size, it's weight is divided by 8. It gets a +2 size bonus to Dex and a -2 size bonus to str. It would have a reach of 5 ft. Any gear its wearing would change with it and all of it's natural attacks would be reduced by 1 size category. It also gets a +1 size bonus to hit and to AC.
So, it looks like you got everything correct, but you were missing the bonus to hit and AC. I don't know what kind of dinosaur you're looking at but it if doesn't gain any new abilities a cheaper option would be to just forego the size change.
As you gain levels, your animal companion improves as well, usually at 4th or 7th level, in addition to the standard bonuses noted on Table: Animal Companion Base Statistics. Instead of taking the listed benefit at 4th or 7th level, you can instead choose to increase the companion’s Dexterity and Constitution by 2.
It may result in a weaker mount but it could be worth checking out. Also, it might be your only option if the DM disallows Reduce Animal to be made permanent.
Strictly speaking your mount can't be targeted with a Reduce Person spell.
Unless it is an animal companion or equivalent with the Share Spells ability, which lets spells ignore type restrictions.
Strictly speaking your mount can't be targeted with a Reduce Person spell.Unless it is an animal companion or equivalent with the Share Spells ability, which lets spells ignore type restrictions.
It is true that the share spells ability allows the character to bypass targeting limitations. However, the OP stated that they were a Saurian Champion Cavalier and cavalier mounts explicitly lack the share spells ability.
I have no issue with keeping my mount medium I want some of the other listed abilities. I was thinking of the Pachdyderm. The ram headed one if I misspelled it. It gets an ability called Powerful charge that does some damage and stuns creatures hit with it. It charges using the ability getting me into melee then afterwards we fight as a team with my Monk flurry abilities coming into play.
Is there any reason you don't want the mount becoming large? If it's because of squeezing issues you can have your mount take the feat narrow frame
