Madcookie Deckling

I was just about to write a response but cdkime beat me to it (grrr :D). Also while all he wrote is correct, he missed the fact that drawing a card for the turn doesn't use the stack, which means at the start of the draw phase your opponent will first draw his card (unless another effect prohibits this, say he has a Necropotence which btw will negate the combo entirely), your Nekusar will ping them and cause them to discard 2 cards and then since you are the owner of both Teferi's Puzzle Box and Nekusar, the Mindrazer(the part about the extra draw) I belive that you can order them in whatever order you like.

Ok, now I see my mistake in the first comment. What I meant was "placed", instead of "resolved". Thanks, @Kogarashi

Yes, but in my example there are no other ETBs and I didn't specify whos turn it is because it doesn't really matter with just one trigger. I just mentioned the APNAP order to be thorough. Also just to clear any remaining misunderstandings. We assume its your turn. You cast Show and Tell. Both you and your opponent put Ravenous Chupacabra from your hand. Both chupas enter at the same time, targeting each other. Their triggers are placed in APNAP order which means since it is your turn your Ravenous Chupacabra will die first due to your opponents chupa targeting it, then your trigger will resolve killing your opponents chupacabra.

In any case the main question was can Ravenous Chupacabra "see" the opponents creature when they enter at the same time to target it? Yes.

Yes, since by the creatures entered at the same time. Order is this.

1. Say Show and Tell resolves with both of you and your opponent selecting a card.
2. Opponent puts his card and you yours at the same time say Rampaging Baloths and your Ravenous Chupacabra. This means both creatures are on the battlefield when their (if any) ETB triggers activate, which in turn allows you to target their creature with the chupa.
3. ETB Triggers are resolved in APNAP order (active player-non-active player). Since in this example there are no other ETB except the chupas you target the opponent's baloths and if no other actions are taken you destroy it.

I've never played with any un-cards, but generally whenever a text on a card refers to a card by its name it is translated in the cases of copying as "this permanent" rather than the printed name.

706.10b A copy of an ability has the same source as the original ability. If the ability refers to its source by name, the copy refers to that same object and not to any other object with the same name. The copy is considered to be the same ability by effects that count how many times that ability has resolved during the turn.

For example if Mirage Mirror died while copying The Scarab God it will get returned to its owner's hand at the end of turn, regardless of the fact that it is not considered a creature or The Scarab God while in the graveyard. So Phoebe, Head of S.N.E.A.K. will permanently have the text and refer to herself in all instances of the name Modular Monstrosity in her newly acquired text box.

As for the other question I can think of Pongify, Beast Within and Polymorphist's Jest.

Cheers!

Absolutely. They way convoke is worded tapped creatures can pay for or 1 mana of their color so just like you say tapping 5 creatures (minimum one of them has to be white of course) will make Venerated Loxodon practically free, but technically his CMC will still be 5 as normal, in case your opponent wants to Disdainful Stroke it.

It is an old card and the wording is funky. The actual text from Gatherer states ":Target creature you control with toughness 2 or less gains flying until end of turn. Flip a coin at the beginning of the next end step. If you lose the flip, sacrifice that creature." and features the magical delayed trigger keyword "at the beginning of" which does make it a delayed trigger and all instances of it would trigger, regardless of the fact that the original target may or may not have been already sacrificed (due to lost flip) or otherwise removed. It is because of the way the ability is worded.

Flip a coin at the beginning of the next end step(this resolves first). If you lose the flip, sacrifice that creature(this resolves after).

You always flip a coin, but if you lose you must sacrifice the creature. Therefore you can include it in the coin flip deck :)