The Implausibeast does not have game statistics, technically, since it does not need them.
What does the Implausibeast look like? It is hard to say, since each incarnation of it seems to be different. You should go through your collection of miniatures and find the most ridiculous, strange, and unused mini you can find. Maybe it’s for a monster from a completely different game entirely. If you don’t use minis, browse the internet to find some very strange fantasy artwork.
That’s what the Implausibeast looks like. At least when your players encounter it.
When the Implausibeast is encountered by an adventuring party, and not a moment before, flip through your manual of monsters or other reference tome, and select a monster of more or less appropriate level range that you have never really gotten a chance to use. The Implausibeast copies the statistics, attacks, and abilities of that monster.
You may have to exercise some creativity in how those attacks are structured, especially if the mini or artwork you’ve chosen has dramatically different limbs than the monster profile you select. Perhaps the Implausibeast will sprout more limbs, or use its body in an unexpected manner.
Should an Implausibeast survive an encounter, its abilities reset. If it is encountered again, perhaps even by the same group of adventurers, it will mimic the abilities of a completely different monster.
The dread Numeravore is even stranger, but man was not meant to know about such things. Only my Patreon Backers have access to such forbidden lore.