What is the most primitive creature having a memory?

Define “memory”. If you think “react differently to the same stimulus as before not because of sensory or motor fatigue” requires a memory, single-cell organisms may qualify (Google “habituation in single cell organisms”, and find, for example https://en.wikipedia.org/wiki/Habituation, https://www.sciencealert.com/this-single-celled-animal-can-m...)

    https://en.wikipedia.org/wiki/Nematode

    The word nematode comes from the Modern
    Latin compound of nemat- "thread" (from
    Greek nema, genitive nematos "thread," from
    stem of nein "to spin"; see needle) + -odes
    "like, of the nature of" (see -oid).

Rock has memory and nematode worms live in Earth's lithosphere.

aplysia exhibits low level learning, indirectly evidencing some sort of memory

https://en.wikipedia.org/wiki/Aplysia_gill_and_siphon_withdr...