I had some ideas (based on existing official rules) about how costs can target, although it will probably not do what you intended in this case.

Static abilities cannot target anything (only objects in the stack can have targets), so in this case the target is the target of the spell instead (which is the corresponding object in the stack).

If a spell or ability has one or more targets and all of them are invalid when it tries to resolve, then the spell doesn't work, and will go to the graveyard. (See rule 608.2b)

In this case, the target will no longer be valid once it is destroyed, so it will cause Halfdragon to go to the graveyard from the stack. I think you cannot pay the cost if the target creature has indestructible (since indestructible means a permanent can't be desrtoyed), but you can target a creature that regenerates (since that is a replacement effect instead, which does not preclude costs from being paid) and the target will remain in the battlefield and Halfdragon will resolve. (Halfdragon will also resolve if the target is changed before it resolves; the new target will not be destroyed, because the costs will have already been paid by that time.)

Another issue with this is that the wording would seem to mean it has a target even if you choose not to destroy the target. (Rule 702.33g causes a spell with kicker to not have targets that only apply when it is kicked, but that is not the case here. Since the rules do not expect costs to target, there is not a rule relating to the case where an optional additional cost has a target, so maybe it is not entirely clear.)

Costs can't target.

A 2/1 haste Ravenous Chupacabra for 1B to life feels bonkers for 60 card formats.

You can still keep track of it even if the card does not explicitly say so; the marker you use to keep track of it is not itself a part of the game, though.

Permanently? That kind of thing is very annoying to track without a token or aura

­Rod of Ruin was to cast and to activate, but then again it was never really that great.

It should be noted though that not having a mana cost at all associated with the activation is a meaningful boost to the strength of this kind of effect.

NR: I think this could still do with some more juice. Options a. +2/+2 for each creature that shares a type b. Also give an ability (ideally some form of evasion)

Probably not mythic

Is a 4 mana uncommon a bit less overpowered?

2 mana noncreature source which can ping each turn is deceptively strong and oppressive to the point of format warping (ala how for example bowmasters has been pushing out classics such as bob from legacy top tier status).

NR: Love the flavour! I think a catapult ought to kill the creatures it gives flying at the end of the turn (or at least, do some damage to them. Lean into the flavour!)