"Do you have a cat?"
Translation:Oes cath gyda chi?
"Oes gynnoch/ganddoch chi gath" would be the correct sentence structure for that.
That means "Are you getting/having a cat?"
The classic example to show the difference is:
Mae gen i fabi = "I have a baby" (in my arms/at home)
Dw i'n cael babi = "I'm having a baby" (I'm pregnant)