A pseudomorph is something that takes the form of something else.  Pseudomorphs in agate are quite rare but do occur in nodular agates from various locations, usually as a calcite or aragonite replacement.  The Coyamito agate deposit in Chihuahua Mexico produces more pseudomorphs than any other agate location.  
     The formational process goes something like this.  First crystals grow in the gas cavity left in the volcanic andesite rock (these crystals are believed to be aragonite in the case of the Coyamito Agate).  The agate then starts forming, coating the crystals and the inside of the cavity.  The more agate that forms the thicker the coating on the crystals.  This process can continue until the gas cavity is filled or, more frequently stops, leaving a hollow portion in the nodule.  Like all nodular agates, quartz or agate may form changing from one to the other.  
     As the agate layer forms over the crystal, the shape of the resulting formation gets smoother, until it becomes roughly spherical in cross section.  A solid agate cut across such a formation would show a concentric banded pattern almost spherical on the outside and hexagonal toward its center (hexagonal at the point of first coating the crystal).  At some point in the agate formation process the aragonite crystals dissolve away.  In most cases this leaves a hexagonal cavity in the agate where the crystal was.  In some cases this cavity can also fill or partially fill in with agate or quartz.  This somewhat later formation of agate can be banded like the coating of the crystal or have a moss or lacy pattern.  
     Sometimes the formation of the agate will cover a crystal attached to the cavity wall leaving a perfect mold of the crystal in the agate exterior surface.  Calcite molds on the outside surface of nodular agates from many different locations are quite common.  The pictures below illustrate some pseudomorphic patterns that occur in the Coyamito Agate.