I suppose, it probably came from Greek τέρας 'wonder, marvel, monster' and is a possible cognate of Greek θηρίον 'wild beast, monster'; cf. OE deor (Eng. deer); also dragon (Lat. draco; Gr. δράκων). The connection between Gr. drakon and δέρκομαι 'to see clearly' doesn't seem plausible enough, because there is Gr. δορκάς/ζορκάς 'a kind of deer', which could be directly related to 'deer' and deor/Tier; cf. Gr. ταύρος 'bull', maybe even including Slav zver, tur (Old Slav. тоуръ) and Lat ferus;
The suffix -sko (ski, ska) seems to be common among the Slavic and East Romance speakers (possessive nouns).
***R It's all over Romance, see Spanish, Portuguese and Italian -sco, French -sque BUT it's usually attributed to Ligurian