Trygons are basically super-raveners. Once upon a time, they were Gargantuan Creatures, but they've been downgraded to MCs with the 5th edition Tyranid Codex. Their gimmick is that they can deep strike onto the battlefield by tunneling, and can (under certain circumstances) allow other units to enter the board from their tunnels' exits. Trygons are equipped with four gigantic scything talons and a bioelectric field, which shoots lightning at their enemies. There is also a Synapse creature version, the Trygon Prime, which has a more powerful version of the bioelectric field biomorph.
On the Tabletop
The pre-5e Codex Trygons weren't too different from what they are now besides having access to some 4e biomorphs that no longer exist and not having access to biomorphs that showed up later, having gargantuan creature rules, and having Toughness 7 and 5 wounds to the new Trygon's 6 of both. As far as bio-titans went, the Old Trygon was quite modest, ranking below even the Heirodule in all of it's "worse statline than a wraithknight in assault" glory.
In the new book, the main change to Trygons is that Scything Talons no longer let them reroll hits, but it instead gets an extra-attack. Some biomorphs got a bit cheaper too, tail weapons are a thing again, but overall not much was changed. The scything talon change is largely agreed to be a downgrade since Trygons already had plenty of attacks anyway. Trygon primes though, are now capable of taking Bio-Artefacts. Which would be great if Bio-artefacts weren't a load of overcosted "eh". The main ones you're likely to see are the ymgarl factor for a 2+, a miasma cannon for one more thing to do when popping up, and the reaper for the initiative and strength boost and that small chance of instant death. These will quickly drive up the price though, and Trygon primes are expensive to begin with.
Though the Trygon arguably began the trend of "enormous oval base models in standard 40k" when they first appeared, they and the Tyrannofex have largely been overshadowed by later enormous oval base models like the Dreadknight, Riptide, Wraithknight, Tomb Stalker/C'tan Shard, and Imperial Knight. A Trygon will get massacred by a Wraithknight it can barely wound (especially with a shield), get its face beaten in by a Dreadknight or C'tan shard, had better pray the Imperial Knight doesn't roll well with its number of wounds per hit and stomps, and will have an ass of a time catching the Riptide in the first place.
To get the most out of the Trygon, it needs to come with support. Catalyst for FNP, Venomthropes to protect it against shooting, models to join it when it emerges (or they would if the rules for the tunneling mechanism weren't broken), other targets for threat saturation and so on. Recommended biomorphs are Toxin sacs (because you need to make sure you throw down as many wounds as possible) and Regeneration (because you need it to survive as long as possible), with Adrenal glands being a bit more touch and go.
8th edition is here (ish) and Trygons are tasty. First revealed was the changes to deepstrike. Trygons now appear without scatter anywhere that is no closer than 9" away from the enemy. They can also bring a troop unit with them which deploys at the same time, next to them, with the same 9" stipulation. They have the same profile as they do in 7th except their wounds are doubled, they gained a point of strength and they move 9" (movement, ws and bs decay with damage). As for it's melee power: scything talons now reroll 1s to hit again, if you have more than 1 pair you make an additional attack... and the massive version of the Trygon has AP-3 and d6 damage.
"That's a nice tank you have there. Would be a shame if something happened to it."
|Leader Organisms:||Broodlord - Hive Tyrant - Swarmlord - Tyranid Warrior|
|Small Creatures:||Gargoyle - Genestealer - Hormagaunt - Ripper - Termagaunt|
| Biovore - Hive Guard - Lictor - Neurothrope - Pyrovore |
Ravener - Tyrant Guard - Venomthrope - Zoanthrope
|Monstrous Creatures:|| Carnifex - Dimachaeron - Exocrine - Haruspex - Malanthrope |
Maleceptor - Mawloc - Tervigon - Toxicrene - Trygon - Tyrannofex
|Gargantuan Creatures:||Cerebore - Dactylis - Hierodule - Malefactor - Nautiloid - Viragon|
|Flying Creatures:||Harpy - Harridan - Hive Crone|
|Bio-Titans:||Dominatrix - Hierophant - Hydraphant - Viciator|
|Other Organisms:|| Bio-Weapons - Cortex Leech - Mieotic Spore - Mucolid Spore |
Mycetic Spore - Spore Mine - Sporocyst - Tyrannocyte - Zoats