If it is clear which dogs we are talking about it is possible to omit the demonstrative pronoun. Beware, the situation is often not clear and two native Czech speakers will not agree if it is possible to omit or not.
Since it is possible the person who says that assumes (knows) the person being talked to is aware what dogs he/she is referring to, "ty psy" should by accepted as well.
It's been a long time since I've practiced my Czech, but I seem to remember that the direct object of the sentece was 4th case. Wouldn't that make it psov rather than psy?