Your opponent may be protesting at this point “judges can’t give strategic advice!” but it turns out that according to the rules, there’s only one legal answer to this question. So let’s walk through it.
When the combat damage step begins, Thragtusk and Boros Reckoner deal lethal damage to each other. The Reckoner’s ability triggers (but doesn’t go onto the stack yet!). Then state-based actions are checked, Thragtusk and Reckoner die, and Thragtusk’s ability triggers. After this check, any abilities that have triggered but are still waiting to go onto the stack finally get put onto the stack, and because abilities that are controlled by the active player (your opponent, who’s attacking) take precedence, the Thragtusk’s ability will be put onto the stack first (even though it triggered later), followed by your Reckoner’s trigger.
Here comes the most important part: when a trigger that has a target is put onto the stack, you have to choose a legal target for it right away, if able. Because the Beast-creating trigger is still waiting to resolve, the Beast doesn’t yet exist and isn’t a legal target for the Reckoner’s ability. Thus, our study of deep and comprehensive rules comes to a simple end:
“No.”
As an aside, if you were attacking and your opponent blocked with the Thragtusk, it wouldn’t change the answer, either; the Reckoner’s ability would go on the stack first and have to choose a legal target at that time. It doesn’t matter that the Thragtusk’s ability will have resolved and created a Beast token before the Reckoner’s ability actually deals damage, because it’s not there when you choose a target.
Today’s Rules Tip written by Jen Wong