MP uses "telencephalon" to refer to the embryonic structure:
MP:0000787 ! abnormal telencephalon morphology [DEF: "any structural anomaly of the paired anteriolateral division of the embryonic prosencephalon plus the lamina terminalis from which the olfactory lobes, cerebral cortex, and subcortical nuclei are derived"]
However, it is somewhat common to use "telencephalon" to refer to the adult structure. E.g.
synonym: "cerebrum" RELATED 
synonym: "telencephalic vesicle" RELATED 
relationship: part_of MA:0000170 ! forebrain
here "cerebrum" is related.
The adult cognate in MP is called "cerebrum", which is fairly common:
name: abnormal cerebrum morphology
def: "any structural anomaly of the largest part of the brain, derived from the telencephalon, and is composed of a right and a left hemisphere each which contains an outer cerebral cortex and a subcortical basal ganglia; cerebral functions include sensorimotor, emotional, and intellectual activities" [MESH:A08.186.211.730.885.287]
synonym: "abnormal cerebral hemispheres" EXACT 
synonym: "cerebrum dysplasia" RELATED 
is_a: MP:0000787 ! abnormal telencephalon morphology
There is nothing inherently wrong with this scheme, but maybe there is the possibility for misunderstanding?
Perhaps a syn and comment could be added to MP:0000787
syn: "abnormal embryonic cerebrum morphology"
comment: this terms refers to the embryonic structure
Some ontologies such as EHDAA2 use "future telencephalon" for the embryonic structure. This is kind of ugly, but it at least signals the meaning clearly