Pferd is neuter. It is indeed in the accusative case in this example, but ein is neuter accusative. Einen is masculine accusative.
Here's one reference: http://german.about.com/library/weekly/aa033098.htm
Mein Onkel hatte ein Pferd. The conjugated verb, hat, switched to the past tense, hatte.
Reference (for more on the topic): http://german.about.com/library/verbs/blverb_start.htm
If you want to specifically see the past tense of haben (to have), go to the reference, click on haben -> past tense. Repeat for any other verb you want to investigate.