"The form of the English name (since 1698) was altered to its "-goose" ending by folk-etymology. It has no etymological connection with the word goose. Historically, it has also been spelled "mungoose". The plural form is mongooses, or, rarely, mongeese." Source: https://en.wikipedia.org/wiki/Mongoose (I know...not reliable but eh good enough)
One of the OG Epidemic members taught me that it is "Mongooses", forgot who it was but I will admit I learned something that day.
Mongen. Mongi is the singular. Kidding, in the animal kingdom it's mongeese. However in a Bungie update during H3 said because this is the name of a vehicle and not specifically that animal its mongooses.