15 vermin HD, let's go.
Bonespears are your typical fight-and-forget random encounter with-a-twist. They're large sized, with mediocre physical stats and terrible mental ones, some extra abilities in fast healing 5, fire and sonic resistance 10, and SR 23, and a few natural weapons.
Said natural weapons are also its main gimmick. The 2d6 bite is unremarkable, but the two 2d6 horns are harpoon-like and can be launched up to 60 feet away. If they hit a target, they poison it (2d4 dexterity or strength damage, depending on the horn), and impale it on the horn. This deals the damage of a horn attack every turn, and applies cumulative penalties on nearly all d20 rolls. It also deals some extra damage (reducible with a Heal check) when removed. Additionally, the bonespear can use some kind of reverse bull rush to draw impaled targets closer.
That's where the neat things end. If a horn misses, reeling it back in takes a full round. If it hits, a target can simply sever it with fifteen points of slashing damage, in which case the per-round damage from impalement goes down to a mere 1d6. Aside from its two horns, the bonespear can't really attack meaningfully, and its HD and body type make it hard to benefit from a conventional class. Also, if you want to read the rules literally, a creature is in no way restrained when impaled, and can just walk around as it pleases (even a slightly more reasonable reading should still let it move to some extent).
On top of all that, consistent damage, poison, and limited mobility control aren't even that great. They're neat, but a typical rogue is going to be doing all three of those things better, and the rogue typically won't be retrieving their weapons every other round.
While I like the bonespear's concept, its numbers are too low and its abilities too niche to warrant anything but -0 LA.