Honestly, I feel like the seryulin was deliberately made to be as expectation-subverting as possible. Giant dumb slug mounts? Nope: they're intelligent, threatening, and as fast as some horses. Sadly, one expectation they don't subvert is that of HD. Seryulin have ten of them to deal with, and to make matters worse they're aberration type.
The slugs have reasonable stats. 20 strength, 8 intelligence, and +2/+4 everywhere else. They enjoy a considerable +10 natural armor, as well as DR 5/piercing or slashing and acid resistance 10. A seryulin's natural weapons are a secondary bite and two primary slams that deal honestly underwhelming damage for a Large CR 7 creature (1d6 and 1d8, respectively).
First of their special attacks is Pain Toxin. According to the description: "Seryulin riders take advantage of the painful toxin secreted by a seryulin’s cilia to wear down foes. Once a victim has been struck by the cilia, the rider typically withdraws his mount for a round or two (into the water if possible) and lets the toxin do its work". The problem with that: the poison lasts for one round, and deals a whole 1d6 damage during that time.
The other, less underwhelming special attack, is Sticky Spray. ( ͡° ͜ʖ ͡°)
This ability entangles and immobilizes nearby creatures, and even if they escape (which takes a moderately hard strength check or a reasonable slashing attack) they're stuck moving at half speed for a few rounds. That's not a bad ability at all, but the fact that it's 1/day, non-friendly, and somewhat behind the curve of BFC does worsen it.
Further seryulin abilities are the descriptively named "Can't Be Tripped", as well as Slick: a nonmagical Freedom of Movement (except for the part where it makes grappling a seryulin harder rather than impossible).
If the seryulin had had a few less RHD, I might seriously consider them for +0. As they are now, however, I feel like the combination of bad RHD, lackluster abilities, body slot problems and advancement struggles leaves them deserving -0. Do discuss.