A Myriad

1. (historical) Ten thousand; 10,000 [from 16th c.]
2. A countless number or multitude (of specified things) [from 16th c.] Myriad - Wikipedia

Usage notes

Used as an adjective (see below), 'myriad' requires neither an article before it nor a preposition after. Because of this, some consider the usage described in sense 2 above, where 'myriad' acts as part of a nominal (or noun) group (that is, "a myriad of animals"), to be tautological.