There are two different Thereus in Greek mythology.
Another one attended the Lapith king Pirithous' wedding with other Centaurs and was killed by Theseus.
"Thereus, also, who used to take bears on the mountain slopes of Thessaly, and carry them home angry and alive."
Metamorphoses by Ovid
The other one was killed by Heracles when he tried to steal the wine of Pholus.
Greek Mythology Link