example code here

(s/defschema Dog
  {
   :food s/Str
   :activities s/Str
   })

(s/defschema Cat
  {
   :name s/Str
   })

(s/defschema Animal { s/enum [Dog Cat]}) 

then (s/check Animal {:food "biscuits" :activities "running"}) gives error

> Execution error at schema.core/parse-sequence-schema (core.cljc:940). > [{:food java.lang.String, :activities java.lang.String} {:name java.lang.String}] is not a valid sequence schema; a valid sequence schema consists of zero or more one elements, followed by zero or more optional elements, followed by an optional schema that will match the remaining elements.

Any idea what is that I'm doing wrong here?

s/enum is applied incorrectly in the example, its correct application would be (s/defschema Animal (s/enum Dog Cat))

However that won't help in this case - checking enum's docstring:

schema.core/enum [& vs]  A value that must be = to some element of vs.

You'd need to use s/conditional:

(s/defschema Animal (s/conditional (every-pred map? #(contains? % :name)) Cat :else Dog))

(s/check Animal {:food "biscuits" :activities "running"})
=> nil
(s/check Animal {:name "Misty"})
=> nil

(there are also a few other similar options: s/cond-pre could be used if the type of the datums were different (but here both Dog and Cat are maps). There's also s/either but that's deprecated due to it not working good with coercion, so s/conditional is suggested in its docstring