|The obvious Tyranid equivalent of a Grot|
Who are my Terminators?
The obvious answer is Monstrous Creatures, more or less as a whole. In particular, I would say Flyrants (Trygons, Mawlocs, Carnifexes, and Harpies to an even lesser extent, and in that order). The reason I pick out Flyrants first is because while they tend to garner a lot of attention and can do a lot of damage, they also tend to die. While for a typical Terminator unit, you're probably expecting them to die after causing havoc, and plan accordingly, I find a Flyrant to attract as much or more attention, be able to cause less damage, generally work best in pairs and cost loads of points, and potentially give out a Slay the Warlord victory point. While the only real anti-air unit Tyranids have, there are some serious drawbacks to fielding a Flying Hive Tyrant, first and foremost being that in many games you are essentially conceding a victory point.
Other MCs are also like Terminators in that their role is largely to cause damage and attract attention, not to capture and hold objectives. They are good at this role (in varying degrees) just like Terminators. In fact, I would argue you need at least one (other than a Tervigon) to serve as a distraction, cause target priority issues, and generally help dictate your opponent's actions. This is the reason I almost never deep strike my Trygon - he is meant to be a huge bullet magnet, particularly Turn 1, who is fast enough to cause problems Turn 2 or 3 if he is not dealt with.
Outside of HQ and MCs, Zoanthropes, Doom, and Hive Guard can also be seen as Terminator-like. They can cause serious damage and be a huge concern for opponents, but when you break it down, will not contribute directly to victory in 5 out of 6 games.
Now, who are my Grots?
Starting with an HQ choice, a Prime is a bit like a grot - his damage output and threat is practically negligible compared to a Flyrant, Walking Hive Tyrant, or Swarmlord. That said, this makes him much less of a target. As an IC, he is a little more survivable despite having fewer wounds and lower toughness since he can hop from unit to unit, always keeping plenty of ablative wounds handy. I would argue he is more versatile as well; he can go forward to support the assault elements of your swarm, or, my preferred use, hang back, providing additional strength to the backfield as well as keeping synapse for your "Stay at Home" units.
The remaining obvious choice for Grots is Termagants, and to a lesser extent Hormagaunts. Termagants are lowly, not very threatening (at least when vanilla), and are usually your primary means of controlling objectives. I make the argument that Hormagaunts can be considered as well, mainly because they are fast and still lowly enough to be ignored at times (if you have enough Terminator units in your opponent's face) and can capture or contest upfield objectives.
Tyranids have an interesting unit in the Tervigon, who can in some regard, be a Terminator unit, but she can capture objectives and produce more Grots... err... Termagants. Because of her size, threat-level, and general value, she's never going to pass as a lowly Grot. And while she can be as destructive as a full squad of Assault Terminators (Crushing Claws and a timely Warp Speed) or as tough or tougher (Iron Arm), she's generally not as expendable.
This is where the Terminator units show their highest tactical value - in keeping a lynch-pin unit like the Tervigon alive by posing a large enough threat that they must be dealt with or punishing your opponent if they are ignored.
So what are your thoughts? Any Tyranid units you think should be argued for either as Grot or Terminator? What about in different armies? Who are your Grots and Terminators?