The only free and comprehensive online etymological dictionary of the Spanish language
-ido Adjective-forming suffix indicating a tendency.
A 13th cent. learned borrowing from Latin -idus 'id.' From Proto-Italic *-idos 'id.' From Proto-Indo-European *-i̯-dho- 'id.' An i-stem adjective to form an adjective from verbs.
Asturian -íu, Portuguese -ido, French -ide, Italian -ido, Romanian -ed

"Few Romance dialects display denominal adjectives which formally or semantically correspond to the Spanish -ido.... In genetic terms, -ido here is not a derivational suffix but rather a true participle marker. After the demise of [a] source verb in -ir, speakers would have no trouble in peeling off -ido as a recognizable morpheme." ~ S. N. Dworkin, Etymology and Derivational Morphology: The Genesis of Old Spanish Denominal Adjectives in -ido (1985)