I incurred a capital loss (<$3000) more than 5 years ago. I forgot to claim that loss. Can i claim it in 2016 tax return? What documents/forms do i need to submit?

If the loss was 15,000 or greater you may be able to deduct some of that loss on the 2016 tax return. You could amend the 2014 (up until the deadline) and the 2015 return to claim a refund for some of the deduction that went unclaimed as well.

Capital losses must be deducted (or reduced)by (at least) 3,000 dollars per year regardless of whether you take the capital loss deduction or not. Therefore a capital loss of 15,000 dollars would only be deductible for 5 years. (15,000/3,000 dollars=5 Years).

@TurboTaxPhillip : If l lost say $2500 in losses 10 years ago , that i did not claim until now. Can i claim it now? Do i need any sort of documentation that i need to attach?

Unfortunately no. That loss would have had to have been claimed on the return right after the loss was incurred. If there is a balance due on that return from 9-10 years ago, you could amend to reduce the balance due.