sponte

Sponte is technically the ablative of a noun spons, spontis, but that noun is defective to the point of being a ghost: only the ablative sponte and the genitive spontis are in real use, so learners meet it as an adverb and may as well treat it as one.

It nearly always carries a possessive adjective agreeing with the subject: sua sponte, mea sponte, nostra sponte. Dropping the adjective is possible in verse but unusual in prose, so if you write sponte alone it will read as thin. The sense covers both "voluntarily", of people, and "spontaneously, without external cause", of nature, and the second is where English spontaneous comes from. Ultro overlaps but adds a note of going further than expected, sometimes of taking the initiative in a quarrel.