If Rhynos is a multi trigger effect "just like Strike Ninja" then how can it miss it's timing?

Because it can only activate his effect in a chain to a Trap Card. Multi-Trigger is Multi-Trigger.

Strike Ninja is a Multi-Trigger just like Horus the Black Flame Dragon LV8, but Horus LV8 can miss timing to, i.e., monsters that become Spell Cards.

Because Strike Ninja doesn't have an activation requirement. Winged Rhynos does.

Then say its just like Horus LV8's effect if that makes it clearer for you ... That's another multi-trigger event and it has to be chained to the spell, just like rhynos has to be chained to the trap.