I'll take a stab at this. It is harder to critique monsters if you aren't following the format in the MM or the online SRD (see here http://www.d20srd.org/index.htm), because it lists everything by line.
Here is one with a complete stat block. I purposefully kept their advancement lower than the triceratops' advancement, but feel free to change it. If you decide to keep the neck guard (see below)l, change CR to 7. Otherwise, it looks really good.
Linebreaker Lizard
Large animal
Hit Dice: 10d8+50 (95 hp)
Initiative: -1
Speed: 30 ft. (6 squares)
Armor Class: 17 (size -1, Dex -1, natural +9), touch 8, flat-footed 17
BAB/Grapple: +7/+18
Attack: Gore + 14 melee (1d10+10)
Full Attack: Gore +14 melee (1d10+10)
Space/Reach: 10 ft./5 ft.
Special Attacks: Powerful Charge, Trample (2d8+10)
Special Qualities: Low-light vision, scent
Saves: Fort +12, Ref +6, Will +4
Abilities: Str 24, Dex 9, Con 21, Int 1, Wis 12, Cha 7
Skills: Listen +8, Spot +7
Feats: Power Attack, Improved Bull-Rush, Knock-Back (Races of Stone p. 142), Improved Overrun
Environment: Any warm land
Organization: Solitary, family or herd (5-12)
Challenge Rating: 6
Treasure: None
Alignment: Always neutral
Advancement: 11-15 HD (Large), 16-30 HD (Huge)
Level Adjustment: --
I noticed that you did not mention the neck frill. Granted, there is some debate on how the creature used it, but it could be used defensively if someone were attacking from the creature's rear. This is how an alternative version used them (I would like to give credit for this but I've forgotten who the poster was).
Neck Guard (Ex): Styracosaurus has an impressive set of horns lacing its crest. While these are not effective for attacking other creatures, they do give a nasty surprise to any creature foolish enough to attack from the rear or flank from the creature. Anyone who attacks a styracosaurus from the rear or from a flank deals normal damage, but takes 1d6 +3 points of piercing damage.
Debby