Zoat With Multi-Melta
Tyranids in the first edition of the Warhammer 40,000 Rogue Trader tabletop war game.
Tyranids are the exception in Rogue Trader in that they are proper aliens instead of fantasy tropes translated into a sci-fi setting. Mostly. Zoats came from Warhammer.

Rogue Trader (1987)
Exposure to Tyranids tends to cause insanity amongst other races...
Tyranids have two sections in the Rogue Trader rulebook.
Tyranids and the Hive Fleets
The Tyranid threat to the universe is impossible to measure but is potentially very great indeed. Their whole attitude to other races can only result in war wherever they are met, a war which only the Tyranids can win.
Tyranids are described as star-faring locust-like social hive monsters who strip star systems bare.
The "thirteen known fleets" are ruled by a bloated, immobile Hive Tyrant. Hive Lord underlings act in the Hive Tyrant's stead as leaders of the Tyranid warriors.
In rules terms they have the usual base/champion/minor hero/major hero profiles. Wargear is just biological versions of the kit other factions used. Zoats excepted, subordinate creatures are described in vague terms and don't have rules.

Tyranids had one major weakness in the 1980's - a lack of miniatures. There was a picture of a prototype ↗
in the rulebook. It was never released commercially. Smaller models ↗
were included in the 1988 RT3 Flyer; I have not seen these in any other early source.

Tyranids were a major antagonist in early Space Marine articles, but just as background material.
Zoats
Zoats exist because Bryan Ansell, then head of Games Workshop, was keen on the idea of centaur-lizards. They debuted in fantasy Warhammer where Rick Priestly described them as "a bit rubbish" and made their way into 40K.
The Xenobiologers of the Administratum believe that Zoats were created purely for combat. Tyranids, being creatures of space, suffer discomfort if they spend too long on a planet. Zoats, however, are stocky animals that feel quite at home in a wide variety of atmospheric and gravitational conditions.
Space Zoats models came in two parts with four torsos ↗ to choose from and ended up in army lists and prose ↗ .

Rogue Trader defines two types of Zoat, members of the Tyranid hive fleets, and renegades. Renegades could have psychic powers. They were otherwise identical.
What are the chances of a Zoat having a Multimelta?
Rogue Trader has a percentage based equipment system that games masters can use to arm models. Zoats use the Tyranid equipment generation charts.
-
Squads of
4 -
2/3chance of single special weapon in squad -
96-00result onD100for special weapon to be multi-melta
1/4 * 2/3 * 5/100 = 0.83%
An individual Zoat has just under a one per cent chance of ending up with a multi-melta, used for "turning creatures into pools of steaming protoplasm, and vehicles into so much twisted goo."
Genestealers
This enigmatic monster originates from one of the moons of Ymgarl, but has spread throughout space and is threatening to become a real menace.
Genestealers are in the Alien Creatures section of Rogue Trader and completely unrelated to Tyranids.
Eldritch Raiders (1988)
Eldar pirates also make regular usage of mercenaries, but since they feel a natural antipathy towards Orks, and find Humans irksome, their favoured choice are the massive Zoats. Mercenary groups of these weird creatures are very rare, however, and the few groups to be found within the Imperium are renegades from the Hive-Fleets of the dread Tyranids.
-Warhammer 40,000 Chapter Approved - The Book of the Astronomican
The Eldritch Raiders army list has an entry for Zoat Terror Squads. This army is exclusively Edlar pirates, except for the Zoats. One Zoat was guaranteed to have a multi-melta.

Suspensors offset the normal 2" movement penalty for carrying a multi-melta.
Does two bolt guns confer any benefits to the leader? No, you fool! This is Rogue Trader.
Chaos Renegades (1988)
Tyranids and Zoats are included in the Renegade Starting Profile Table on page 233 of Slaves to Darkness along with every other major faction in the Rogue Trader rulebook. This table can be used to roll up a starting profile before their transformation by the corruptions of Chaos.
Chaos Renegades have a slim chance of generating D3 Zoat followers.
Tyranid Army List (1992)
Plastic Tyranids appeared with the release of Advanced Space Crusade . An army list for 40K followed in White Dwarf 145.

This list is presented in the form of Army Cards with double-sided pages - description on one side, rules on the other. It is the only early army list to use this design.

The Tyranid wargear list replaces the usual 40K weaponry with unique bio-engineered weapons - Deathspitters, Boneswords, Fleshborers and the like. There are unique rules centred around maintaining contact with the Tyranid Hive Mind.
To pad out the army list many things gained Tyranid origins. This later became a meta joke where some Xenobiologers where described as overenthusiastic in ascribing the origins of creatures to the Tyranids.
Zoats
The organic weapons the Zoats were modelled with were re-categorised.
Zoats come in units of one for 50 points armed with one of the following:
-
2Flesh Ripper pistols -
Flesh Ripper and Power Fist
-
Barbed Strangler (
S4with36"range)
Barbed Strangler Special Rules: The Barbed Strangler fires a seed pod containing an embryonic Strangler Beast. If the target is wounded and its saving throw is failed then the model is ripped to pieces and killed, no matter how many wounds it has. A full grown Strangler Beast bursts from the torn corpse, hooked tentacles slashing out to capture new victims. Anything within 5" of the first target will be grabbed on a
D6roll that exceeds the range to it. Any living thing that is grabbed is killed automatically, though a new beast is not created. Non-living things are trapped until freed. A Barbed Strangler Beast can only be killed by a hit from a flamer or area effect weapon.
Nasty!
Zoats retain their role as subverters of alien races and allow one unit of Genestealer Brood Brothers per Zoat.
Genestealers
Genestealers became the vanguard of Hive Fleet Kraken sent out to infiltrate alien societies.
Sections of the Genestealer Cult list are included: Purestrains, a Patriarch, a Magus, Hybrids and Brood Brothers.
The Hybrid profile was simplified to a single entry.
Hunter Slayers

These were the RT3 Flyer ↗ Tyranid miniatures cast in a new role.
Slayers are squads of 8 which was inconvenient because they were sold in blisters of 5. There are two metal sculpts. I used a hair dryer to make the tails malleable for a bit of variety.
Screamer Killers
Another metal model, this is the Tyranid equivalent of a Dreadnought.
Squig Swarms
Squigs were an oddity in 40K. They were described at length in the Space Ork background material. They had models thanks to the Warhammer Fantasy Battle game. They did not have rules in 40K.
Long ago the Tyranids started bio-constructing a range of hardy creatures from Ork generic material. When other Orks found out about this outrage they stormed several Tyranid vessels to rescue their compatriots, only to find that they were too late and their compatriots had been changed into little creatures they dubbed 'Squigs'. The Orks recognised the Squigs as Orky and the Squigs seemed to recognise the Boyz - it seemed that a thread of the Orky spirit still remained in the hideously changed creatures and it was at odds with the hive mind. The Orks took back with them as many Squigs as they could find and, with much evolution and adaptation, they have since become a vital part of the Ork way of life.
-Andy Chambers and Jervis Johnson, White Dwarf 145
The above is just part of the Squig Swarm army card text. Squigs get more text and paragraphs to awkwardly justify their inclusion than any other entry.
Mind Slaves
Tyranids are not amenable to having allies. This entry allows Tyranid fans to include basic Chaos Space Marines, Orks or Imperial Guard squads. Unlike in Advanced Space Crusade there is no option for Gretchin.
Named Hive Fleets
Locust and Behemoth are mentioned as antagonist Hive Fleets in WD96 and WD97. Hive Fleet Kraken was added circa WD145.