Latishya has got it right. Pieariansipist might not be a good example since it follows the rules that would be normal for words ending in the suffix -ist. A better example might be mongoose. The temptation is to form the plural as mongeese; this is incorrect but will probably be seen in informal use. Mongooses outgoogles mongeese 170,000 to 34,500. It's more commonly seen in verbs. For example, the verb grandstand has as its past tense grandstanded and not *grandstood. It derives not from the verb stand but from the noun grandstand.

To expand on my peas and cherries examples, they were originally mass nouns, English pease and Anglo-Norman cherise. They were both re-analyzed as count plurals and the singulars were back-formed, giving us the count nouns pea and cherry respectively. Getting back to Legos, given Nuncle's revelation that the correct term for the little thingies themselves is LegoŽ bricks Lego is actually functioning as an adjective. Since we have nouned it we can make the plural any-dang-thing we want.