Okay, on to the comments.
That AC and Dexterity seem a bit high. Presumably it's one of the "heavily armored with spines and tubercles" species as mentioned on Wikipedia's
Trigonotarbida, but still I think they should be less agile than a Spider.
I'd go for maybe AC 15 and either Dex 13 (+1) like a
Giant Scorpion (similarly body type, Large size and decent chitin armor, although it's faster) or at most Dex 15 (+2) like a
Giant Crab (which has the same 30 ft. speed but is Medium).
Would be willing to accept AC 16 like the recent
Giant Solifugid conversion.
Compared to a Giant Solifugid or Scorpion, this critter is a lot slower but has a thicker shell, so it could have the same AC but it's more due to thick chitin than reflexes.
The Bite should have "
Hit: 10 (2d6 + 3)" - presumably the +3 got left out when calculating its average damage.
Apart from that, the "Body Rush" is the only bit I have major issue with.
For a start, to me "Rush" implies it, well, rushes towards a target and runs into them. That doesn't appear to be the case here. Maybe change that to "Slam?"
Don't much care for the phrase "spiny rear" either - I'd prefer "abdomen" to "rear", and it could presumably kick them with a spiny leg instead. Maybe mention both? Actually, that implies it could do piercing or bludgeoning damage, depending on whether the slam was with a spiky bit of its body. Come to think of it, I like the ring of "tubercles" and might work them into the description too.
As a Reaction, the mechanical explanation should start with a triggering condition.
Also, "
Melee Reaction" doesn't appear to be a 5E rule term, it should have a "
Melee Weapon Attack" in there somewhere instead.
I'd think the damage should include its Strength bonus - it is whacking the enemy after all.
It also needs to say what kind of check that "DC 10 or fall over" is made against - I'd guess Strength, like the Bite action of a 5E
Wolf?
Actually, it is basically a Reaction version of a wolf's bite attack.
Putting that together:
Reactions
Body-Slam. If the trigonotarbid has a target at its flank or rear, it can use its reaction to strike that target with its tubercle-covered abdomen or a spiny leg.
Melee Weapon Attack: +5 to hit, reach 5 ft., one target.
Hit: 4 (1d4 + 2) bludgeoning damage or 4 (1d4 + 2) piercing damage. If the target is a creature, it must succeed on a DC 12 Strength saving throw or be knocked prone.
[EDIT]refined the Body-Slam wording with "target" instead of "enemy" in the trigger. Polished the Armour Class musings.[ENDEDIT]
