Can dehydration make your creatinine high?

Yes it can: This is referred to as acute kidney injury. Generally it is reversible with rehydration. We will see the BUN rise first and then if severe, the creatinine will also rise.

Co-tango usually: The BUN is usually 10 times the creatinine. When the creatinine goes up, the BUN goes up similarly to roughly keep this proportion, in most kidney disease states. The notable exception is when the creatinine goes up say by 20% to 1.2, but the BUN goes up *5 to 50. This strongly suggests a pre-renal state, commonly dehydration. Mild elevations of BUN with normal creatinines aren't of concern.